PC and/or a new player is their biggest threat. But they are not as big of threats as many people make them out to be. PC might bring Sony down to 85 million lifetime sales of a PS5 or PS6, but that's about it. Sure, there will be a slow trickle of kids growing up that make the PC their main platform, but the PC is slowly getting worse as time goes on. There are more and more storefronts these days. Windows 11 will likely be $10 to $20 a month to rent, with no way of outright buying a permanent license. As the west continues to decline western consumers will have less buying power than ever, (especially in the USA) making a costly PC a luxury that many can't afford. As India and China rise to the rank of 1st world countries, they will have consumers who will want to play AAA games, and the cheapest entry to that is a console.

Many people think that iPhones and streaming will do the same thing to consoles as they did to portables. But this idea has several flaws. First off the 3DS had a poor launch lineup, and was ridiculously overpriced at $250 for a tiny OG model. If NIntendo had just launched the 3DS with a 2DS XL model for $180 then the 3DS would have reached 90 million lifetime sales easily. The Vita was just a terrible handheld, with almost no exclusives, no backwards compatibility, and proprietary memory. It was Sony's Wii U, and sold poorly more due to being bad, than to having lost to iPhones/Tablets. The original DS was the PS2 of handhelds, and nobody should have expected 150 million lifetime sales to continue forever. Had Sony fully supported the Vita, and had Nintendo launched with a 2DS Xl, then the handheld market would be doing fine to this day.

But anyway, my point is that iPhones/Tablets are not as big a threat to consoles as people think, because they didn't exactly cause the downfall of the portable market. At least not by themselves.

The infrastructure for Streaming just isn't there, and probably won't get there for another decade.

IMO PC/iPhones/Streaming will not really threaten Sony. They will just take the marketshare that was previously held by the XB1. Gaming as a whole has grown since the start of this generation. Xbox sales have been nearly cut in half this gen. But you see Sony selling only 20-30% more PS4s than they did PS3s. So where did the rest of that growth go? Where did most of those Xbox consumers go? Well, they went to PC, and smart devices.

Edit: Consoles have grown more YoY than PC/Smartphones. 

Microsoft because of PC and Xbox.

Don't see Nintendo as competition because they seem to have a different target group.

Nvidia as a technological competitor since their they're trying to move more gaming enthusiasts into their ecosystem
Steam steals their marketshares as 3rd party software distribution platform.
Nintendo as an indirect competitor since, while it's true a Switch and a PS4 can coexist in the same living room, the more time and money a customer spends on different platform the less time and money is spent on yours.
Microsoft can become a big threat if they really manage to really pull off their "netflix" game streaming service.

I mean, in terms of hardware it has to be Nintendo as of now. Nintendo has a home console hybrid competing for living room space same as Sony and Microsoft. So to say "oh no, I put them in a different pool" is quite blind. The age group that buys the Switch the most is comparable to the age group that purchases the PS4. Sony and Nintendo's and even Microsoft's goal is to appeal to all ages so they all have the same target demographic. There was a study that came out that I can't remember at this time that gave percentages of those who own a PS4 and Switch in the US. There's obviously overlap so they do compete for our dollar.

In terms of parity, no question it's Xbox and PC. And by that I mean, around the same target specs, and the most prevalent question: PS or Xbox. But even as Sony has pointed out on a competitive level, they can't ignore the Switch and Nintendo, while they and now Nintendo are smoking the Xbox One. Overall, I say Nintendo, especially now since Sony is losing their grasp on Japan (Yes, I know PS4 is going to outsell PS3 in Japan and displays console growth) but they're no Nintendo over there whilst Xbox is non existent there.

The future is changing quickly so who knows, maybe Google might be Sony's #1 software and hardware competitor...the industry is about to change and quite soon.

It's gonna be Apple..

Microsoft: some might scratch their head at this since X1's sales pale in comparison to PS4's but they're not just XBox but PC as well.

Very enjoyable post. One thing I'd like to mention is that Sony did enter one very experimental area with VR which I'm very proud of them about.

As far as direct competition, to me it's plainly microsoft. As a budget gamer, If I go on to eBay to buy a console, to play xyz multiplat: I can get a PS4 Slim with it's exclusives and massive support, or I can buy a sexy white Xbox One S at half the price. Obviously it's discounted because it isn't selling well. And I say sexy because I like my consoles looking like a storm trooper. But nothing else on the market competes with PS4 on the mass scale. Only PC has the multiplatform games.. but it's going to cost way more than a $150 used system.

Other companies like to fake to news and investors about plans to enter the console wars, they generally to get off the ground. Only microsoft has been able to actually enter and that's because they're microsoft. Plus, MS can get up the next round and go again. People like to say that it's impossible for them to hit a home run but that's totally not true. It can happen. Not saying it's going to, but it can.

Realistically Sony are their biggest threat. If i had to say a company I'd say MS as they have the same target audience but that battle is so easily won by Sony.

In terms of platform the PC is a bigger threat but things are getting worse over there lately. Sony are in a great place and if Google release a console it might finish off Xbox hardware wise but not Sony.

"things are getting worse over there lately"

How do you mean?

I appreciate the OP for making a very thoughtful post.

I've already made a prediction thread about this, but the Switch is going to be the dominant console this time around.  In fact, it will be so dominant that it will be the best selling console of all time.  Switch is already going to take the entire handheld market, and it will get a portion of the home market too.  I believe it will get a majority portion of the home market.

So, the biggest mistake Sony, Microsoft, and Google can make is assuming Sony is the main threat.  Nintendo is the main threat this time around, just like they were the biggest threat with the Wii.  Most likely the other companies will underestimate Nintendo again, which means they will all get blindsided again.