Nintendo won't stick with the Switch forever. They'll upgrade eventually.
This is true, however that will likely be further into the lifeapan of the PS5 and X(2). If they follow the business model for Switch, which makes sense, and release a Switch 2, it will likely have a similar power gap with the other two that we see today. Also by then, the PS5 would likely have a commanding lead to where the damage will have already been done, unless Sony drops the ball elsewhere that is.
Now if Nintendo were to get into the market of launching beefed up consoles, which likely will not happen, but IF they do...things could be a little different. However it would still result in them needing to launch it closer to the PS5 rather than later to make the biggest impact. If they did this, I could see Switch becoming a handheld that can be connected to the TV and officially replacing the 3DS, which would keep them from shutting out millions of Switch owners early.
Now if this happens...well then things get interesting as we would now have a 3rd beefy console for MS to crossplay with. However based on Nintendo's history and how this requires a number of factors to line up...it is still possible...but highly unlikely.
The power gap between Switch and PS4/X1 hasn't stopped games like Doom 2016 from showing up on the Switch. That's the thing about software: it can be scaled.
Because crossplay enables one to play with other people regardless of platform, widens the pool of players to play multiplayer games with, increases players to play different multiplayer modes, shortens matchmaking time, can connect you to more people who are closer to you geographically thus helping ensure more stable connections.
I don't care about the first one at all.
I get that you don't but surely other people do.
As for the rest, I'd like to read about it at the source, if you don't mind. Which Xbox/Switch crossplay games are you drawing this information from? What are the actual numbers? Seconds? Minutes? How is that being measured?
I think it stands to reason: I expect a multiplayer game to benefit from a high player count since this increases the chance that there are players near you to play with as well as more people interested in playing whatever mode you are interested in. I'd also expect a matchmaking algorithm to discriminate based on connection quality and with more players to choose from, there should be less chance an algorithm end up having to pick poorer connections for the sake of starting a game.