Now, I didn't read the OP because it was too long. But Nintendo consoles are like ten times better on average than not owning a Nintendo console

Then they better change this because unless they have luck like with Wii they will fail badly like with Wii U. Wait a minute weren't N64 and Gamecube more like the other consoles by then and they failed anyways?

The problem with Nintendo is a problem of brand when it comes to home consoles, playstation is THE BRAND when it comes to it like Nintendo is when it comes to handheld ones.

"Nintendo" is a pretty huge brand

I have always considered Nintendo not to be a competition to PS/XBox...starting from my beginning of seeking a games console back in 2008, when it was already then visible that it is missing a lot of interesting and exciting titles that the other two are bringing (GTA, RE, Tomb Raider, Borderlands, Battlefield etc.)
That immediately deleted Wii from the list of consoles I'm considering, because I don't like Nintendo games.

And that's basically where Nintendo is, as they are still behaving like it's 1989, aiming their product mainly at children, with their secondary target being Nintendo enthusiasts (former children of the 90s). And that is why its home console is in a steady decline from the onset, dropping in popularity every single generation, with a Wii abberation, when they struck gold with selling a gimmick to non-gamers.

Unless they seriously change their strategy and start targeting regular adults, they will continue to decline, as very few people remain interested in their antiquated offerings.

"steady decline"? whenever one of the big three has the highest selling console of the generation they always have a decline in sales. Did sony have these same problems last gen?

According to VGC numbers:

NES - 61.91 million

SNES - 49.10 million

N64 - 32.93 million

GC - 21.74 million

I call that a steady decline... as I said, Wii thrived on a gimmick and bucked the downward trend, but now that the gimmick is gone, WiiU continues the 25-year decline...

(Also, it's not "whenever one of the big three has the highest selling console of the generation they always have a decline in sales", as PS2 witnessed a 50% growth after PS1 won its generation...)

You could also equate the lowering in sales to much higher level amount of competition. Also all the systems had gimmicks. Theirs just weren't up to the Wii's.

Wii had a great attch ration for mario, wii sports and just dance etc but Xbox 360 actually has 50 or so more million sellers, so does the PS2.  Wii is not part of the argument  anyway, it was a mainstream system, wii U isn't which why is where the guys statement comes into play. 

You can take away key Microsoft and Sony titles away like Halo, GT, Gears, Fable, Uncharted  and you're still left with dozens upon dozens of system sellers. You simply take 4 key titles from Nintendo (Zelda, 3D mario, Mario Kart and Smash Bros) and I have no idea what audience you'll actually be left with on their systems. I don't think the post actually said that people only buy 1 or 2 games, simply that they only pick one up for one and 2 games. I know at least it applies to me, as much as I appreciate Wii U's lessor offerings I would not pick it up without Mario and Zelda. You can take FInal Fantasy XV and Uncharted 4 from the PS4 and I'd still probably pick it up, but thats just my take anyway...

Then theres handhelds

You just described the 3DO. It doesn't work.