"A large part" does not imply it was the main reason but a major contributing factor. How big a part? No one can know for sure, but there's no doubt it really was.
Most of the PS2's success came from strategic pricing, massive 3rd party support (a lot of it due to the incredible sales of the PS1), advertising and continued support after the successor arrived.

Like I said; there's no denying that the PS2 was and is a great console on its own, it was the only one I owned during the 6th gen and I loved it to bits. But, you cannot seriously deny that the things I mentioned helped it greatly along the way. If Nintendo and MS had made no mistakes, it would have taken chunks out of the PS2's enormous marketshare.
Even if you're a market leader and end up selling the most, the competition can put up a real fight if they make some good decisions and you make some bad ones (see the Wii for a prime example of this).

^^ pretty much in line with my thoughts. I love sony but I just think they got way to arrogant at the start of this gen and that's why they got in the position there currently in.

Well, clearly Sony entered the gen with the biggest chance to win... but they more than made some bad moves.

My view on the various wrongs of all notable parties:

Sony launching PS3 with high price point (Cell & BR) and very weak and embryonic online - big mistake, particularly in US. They're recovering but they went from sure thing to last in a blur thanks to almost totally mismanaging the transition from PS2 to PS3, particularly early on. Their poor marketing and lack of imagination in attracting customers also warrants a mention.

MS rushing to market and paying for it (RROD) and overly focusing on what US wanted (online and violent shooters) thus failing to capitalize globally on Sony's mistake (to be fair though their approach did do well in US - still is in fact).

Nintendo underestimating how long HD consoles would remain viable vs Wii due to specs, missing the boat in online and failing to work well with third parties.

Western developers for chasing gritty, violent online orientated titles in a mad rush to gain "some of them CoD sales" resulting in a severe drop in breath of options and style of games (which Nintendo filled to an extent). Of course this worked for some but I see the longer term impacts as being more negative. Also too many developers allowed "must have MP" and the fear of used game markets to influence them, with often terrible results. As many strong SP titles showed, get the game right, market it right and their is still a huge SP market. In fact, what I find is ironic is that the SP market is arguably less competitive right now than MP, allowing more chance for success if you get it right.

Japanese developers for trying to ape Western developers and ending up in the middle with expected results. Special mention for SE who seemed to almost wilfully hurt their brand and franchises with a variety of odd choices both in the West and the East. Capcom also perplexed this gen seeming to be on a mission to annoy their own customers. Also Konami for fucking up Silent Hill, one of my favourite ever franchises. Thank's Konami for thinking their must be some magic formula to keep a title mature, interesting and mysterious and non-combat centric while adding a lot of combat and explaining everything Western horror film syle - now you know better maybe we can get a proper Silent Hill soon? (to be fair downpour, if technically more solid, is a bit of a return to form but still a fair way off Silent Hill 2).

To close, I'll note that while Sony entered as gen as the most likely to win based on brand position it now seems clear that if Nintendo had just pushed a bit more power into the Wii, sorted out online and got Ps3/360 level third party support they'd have scored PS2 big this gen. Not that they weren't successful, but they could have appealed to almost everyone with a few different choices.

On a positive note though I'd say Steam, Valve and my PC did nothing wrong this gen that I could notice.

And there we have it.

I liked the last sentence the best of all, Valve and Steam are the true unsung heroes and winners of this generation, both for me and the market as a whole.

I would say Sony dropped the ball in this generation. They were positioned in pole position prior to the start of the gen, but messed up in several aspects: 1. Launch date was too late, 2. Pricing was too high, 3. Perception of consumer base was wrong.

They got too comfortable, they did so damn well with the PS2 that they thought every PS2 customer WILL buy a PS3. So although the hardware is fantastic, they might have been able to use something a little cheaper on the inside to cut down on price. They really assumed their customers would jump off bridges for them, but they didn't. Many companies that get to the top get too comfortable. If you do, you are going to lose the next round. Nintendo did it after SNES, Sony did it after PS2, Nintendo did it again after the DS (although they have recovered from their mess up with the 3DS, but they were lucky that there was no NEW handheld competition in the market, otherwise, if the PSVita had already been out, chances are many who were turned off by 3DS at the start would have hopped on the vita bandwagon).

EDIT: I need to add that Sony also underestimated their competition. They had no idea Nintendo was going to release the (at the time) unique Wii that disrupted the market, and that Xbox360 would pick up the steam rapidly with it being the only console available for a whole year plus its online innovations that Sony did not have at the time.

It takes a ton of work and innovation to REMAIN in the number 1 spot.

leading everyone with more software support then their competitors combined = bad???

and this was all Sony's doing, but great non the less, no need to waste either of our time

and MS made no mistakes last gen as it was their enrty into the market, they had no base on which to screw up, it's just their effort couldn't match Sony's, and Nintendo's 3rd party and core "mistakes" are the same "mistakes" they made this gen but they were still successful, so realitly would mean that if had they done better last gen it more than anything would have just stopped the gamecube from sucking as hard as it did

I wouldn't say they're unsung, they've achieved a status amonsgt PC games, blogs and news sites that I don't think any other company could reach. Undoubtedly the true 'winners' of the past generation. 

When I think of game companies that did things wrong this gen, developers like Factor 5 and Free Radical immediately spring to mind. These companies were both known for a string of hits like the Rogue Squadron and TimeSplitters series during the previous gen, yet both tried putting their eggs all in one basket so to speak by releasing new IP titles as PS3-exclusives early on in the system's life cycle... Factor 5 with Lair and Free Radical with Haze.

Both of these games bombed spectacularly at retail thanks to scathing reviews ,despite both being hyped considerably before release, and the fallout resulted in both companies essentially going under or dissolving into shells of their former selves.

everything about ps3's launch. Ridiculous price. Horrible gamelineup for god knows how long. PSN was awful initially. Sony banking on bluray exploding like cd and dvd did. Losing so so much exclusive 3rd party support. 

360. RROD fiasco completely ruined the console's imagine, and it would not recover till the complete slim redesign of the console. Focusing way too much on Japan which was a lost cause. It wasted so much money buying up exclusive rights to a bunch of jrpgs. It would have benefited a lot more getting exclusives that catered to Europeans and Americans.

Wii, While you could argue the system could have benefited from being more powerful, I feel nintendo would have done horrible had they gone that route. Being slightly more powerful is pointless. For it to be meaningful the wii would have to jump to 360/PS3 level performance so developers could port games to the wii. Had it done that it would have been significantly more expensive, and quite possibly allow the ps2 to continue being the "casual audience's" console choice.  And the wii would have still recieved the worst 3rd party support because it's control scheme was so different than the other consoles.
I personally think the wii did everything about as perfect as possible. It took a hit in third party support and longetivity in order to be the cheapest console. Its control scheme cost it a lot of third party support as well, but allowed it to captivate a huge new audience.