There's a chance that the Switch 2 could be a Wii U situation. Perhaps not that bad but it's not out of the question. Could also ve a 3DS situation but lord knows it won't be an OG Switch situation.

Sure, but the question was about getting "all" or even really most third party support. I interpret this to mean when we expect most games to come to Switch 2 and it would be unusual if they didn't, rather than have the default expectation that they aren't going to come and are mildly surprised when they do. That likely requires the Switch 2 to be enough people's primary system rather than a system almost every-one has, but has as a secondary system. 

The 1 reason that will make sure Swith2 is not going to sell as bad as Wii U, is because it's their only platform now. E.g. it gets Pokémon mainline games. Previous home consoles never got a Pokémon mainline game. This alone will have it at least sell double/triple the Wii U numbers.

If the Switch2 will be able to reach the first gen numbers, I believe will depend on the market Nintendo calls "Other". I do not think it will reach the same heights in "traditional" markets like US, Europe and Japan. But if markets like China would open up more in the coming years, the ceiling would be raised substantially.

Depends on what it is and how they market it. I just don't think people will buy a Switch 2 when they have a switch. If it's iterative then some people will upgrade and new people will buy in but can't see the majority buying a Switch U nor can I see people buying in masse unless it's something revolutionary. I'd say 3ds numbers would be a good parse, maybe add on 10 million for dedicated Nintendo Home console players. It's not getting anywhere close to the Switch is what I'm trying to say, The OP suggests the popularity will go up but I just want to point out that it's almost certainly going to decline and decline by quite a large margin. 

Where does your certainty come from here? Not saying it isn't a possibility, but it isn't clear why you're certain.

At the end of the day people buy new platforms for the games. If the Switch 2 has the games people want to buy and no other platform does, they have no choice but to buy one if they want to play those games. Nintendo is positioned a lot better when it comes to having exclusive system sellers than other platforms, and they are much less likely to maintain support for the original Switch beyond a few years (unlike what we've seen with the 8th -> 9th gen transition for the other consoles.) 

My prior is that there likely will be a decline compared to Switch, given that it is hard to maintain #1/#2 lifetime platform sales, but I am nowhere near certain it will be "quite a large margin" or "3DS level." 

There is a lot more uncertainty than certainty when it comes to generational transitions. 

My opinion. It's a feeling I get way down in my center, I just know it. There's not much Nintendo can do to market this thing to propose people who have a switch to fork over for another platform. If they don't do BC, they're screwed cause of peoples digital libraries/online etc. If they do BC, why upgrade. They're screwed no matter what angle they come across on this and the best they can hope for is a 3DS situation. Just my prediction, like based off of what I can see what has come before. Nintendo tend to have a track record of screwing up the next system after a successful system too. There's that. 

How are the nearly hundred million people who bought a Switch to play Pokemon, Super Smash Bros, Animal Crossing, The Legend of Zelda, Mario and Mario Kart going to play the new games in those series if they don't buy the new platform? 

We're talking about games series that sell individual titles in the 20-60 millions. 

Nintendo is almost certainly going to have backwards-compatibility for Switch titles, but people still are going to be interested in buying the newest games in those series and Nintendo isn't going to release them on Switch outside of the first-year BOTW/TP like situations. That wouldn't make sense at all. 

This isn't gonna go well if I reply with my opinion and it'll derail the thread. Bowing out here, early. 

Best of luck to the Switch 2 all the same. 

I mean, with an awesome line up with a little help from third parties they have a proven track record. But also they have some struggles as well. Only difference now is they have all their eggs in one basket. If the Switch 2 fails, if they do not handle it right, Nintendo could finally be a third party. There is no handheld or Home console to save their sales. This is their only system. Sure the Switch will continue to push sales for years like the Wii did for the Wii U. But even that will only go so far. They have to treat their fans right, or they will bail on them like many systems before.

I think they Switch has been given plenty of life. They do not need to dump the system right off the bat, but shouldn't do a A bunch more big games for it once the Switch 2 is officially announced.

"Most" third party (console) games probably will come to Switch 2, just perhaps not most AAA games.

Thousands of games release on consoles each year, but of those only a relative few of those are AAA. Indie, AA, and other smaller stuff will be there in force.