Million sellers by publisher

Nintendo-20

Capcom-9

Square-8

Enix-4

Acclaim-3

Bandai-2

Banpresto-1

Virgin Interactive-1

3rd parties were a big part of SNES success but it was primarily 3rd party exclusives that did big numbers, not multiplats.

1. The Wii U likely had the same intentions. While I agree with you that it will likely be a full life console, the only thing that has been proven is intent.

2. The Switch does not have a healthy ecosystem. It is a healthy platform with healthy interest, but an ecosystem that requires a cell phone for voice chat is in no way healthy.

That's because the games are developed for the lowest common denominator (XB1S) and ported up, gaining higher frame rates and  higher  resolution textures. Multiplatform titles for Switch would either require hampering what other consoles can do in the end game, out a separate studio would be required to make a Switch version specifically.

The Famicom/NES, Super Famicom/SNES, and Game Boy were supported for a very long time. Not just in software, but retail availability. This isn't unprecedented. Heck, if the Wii kept more momentum at the end of its lifespan (and the Wii U was not backwards compatible), I guarantee we would still see brand new Wii consoles still available in stores.
To survive, the Switch is going to need a power upgrade at some point. I know people who play the Switch don't care much about fps, resolution, power, etc. but it needs to be improved. The Switch is already absolutely pathetic in performance compared to the PS4 Pro and Xbox One X. It will be annihilated when the PS5 and next Xbox (if there is one) get released.

And what competition did 3DS have? That's how it managed to last this long without being replaced, and, as a consequence, reach the sales numbers it did.

3DS isn't doing worse mainly due to the Pokémon Go effect.

If devs can't port to Switch newer games, how won't that hurt Switch's sales?

I don't know if multiplats is the best way to see it.
In other words, when Nintendo had the best - 3rd party support, it sold the most - or a lot (NES, SNES); when Sony had the best support, it also sold the most (every single console).
The exception to this was, obviously, the Wii.

I'm not saying that if Switch gets every multiplat, it will rain supreme. But if it doesn't it will be a very, very, hard road to the top. And so far, history shows that 1st party games aren't enough.

"Points To The PS1 and PS2"

PS1 and PS2 would be fine with out any first party, Nintendo's has never really relied on third party AAAs, all of their portables success are their own doing, Wii never relied on them and now Switch as well.

SNES wasn't successful because of third parties either, for those of us who were actually around and gaming at the time third parties were trying to jump ship to the Megadrive, the SNES took off because of first party and is the platform that ironically actually cemented Nintendo's strength as a first party developer because third parties were trying to avoid the platform but when DKC arrived and sales shot up through the roof followed by other first party games they were forced to support the platform as the MD didn't do well in the US and Japan.

They don't need an updated system to keep the hybrid concept going games designed for Switch will do that themselves, the mistake you're making is thinking the portable and home console aspects are separate they're not the markets are separate right now yes but the unified concept doesn't have them as separate entities. For example eastern developers will still support the Switch no matter what and every game they release on the platform is still a game built for the hybrid concept.

Fact is people make the mistake that because Sony and MS rely heavily on third parties for success that the same applies to Nintendo when the truth is Nintendo's unorthodox approach means they operate in a way that is not dictated by the tide hence why all the DOA claims against the Switch got blown out the water. For Nintendo what dictates their success more is making an appealing concept and execution.

True.
But, how much of this handheld consumer isn't also a home console consumer?

GCN and Wii had better 3rd party than GBA and DS?

If i'm correct, you're assuming a large part of handheld consumers don't necessarily demand the best 3rd party support in order to buy a handheld. Therefore, they would buy a Switch even if it doesn't have the best support, seein as it has that handheld aspect.

Switch may not be only a home console, but that doesn't mean that people don't see it as such, and, therefore, have different expectations.
If a usual handheld consumer buys Switch, he, most likely, will want to take full advantage of it. Being cut short of 3rd party support might have it's consequences.

It really is a lot about perception and Nintendo's marketing, and even consumer perception, seem to point out to people wanting to be treated to console games, not handheld games.
So, i think it's safe to think that people buying Switch will want the home console treatment, not handheld treatment. And if that doesn't happen, Switch might not leave to it's full potential.