Overall - a fair point - "what happens this year" is a curious thing. Lets have a discussion about this:
1) PS2 sales after the PS3
Don't forget, the PS2 sold almost a third of its lifetime sales after the next generation had started
and after the PS3 launched, more or less, it basically looked like this:
2007 - 14ish million
2008 - 14ish million
2009 - 8ish million
2010 - 7ish million
2011 - 6.5ish million
After - 2-3ish million until it stoped production
Xbox and Gamecube were long gone by then. Majority of it sales during this time were due to hardware refreshes, bundles, a very low and attractive price point, strong sales in emerging markets and - most importantly - Sony's willingness to continue production and dedicating resources and allocations to product lines given it still sold in certain markets. It had a bunch of losses from the PS3 so I can imagine keeping the PS2 in production played a role in maintaining some balancing of the books.
What this means for the Switch is there is the potential to have more sales after the Switch 2 launches (whether it is end of 2024 or as now purported to be early 2025) if Nintendo follows a similar plan with focus on emerging markets, lowering the price point, maybe a cheapo hardware refresh of sorts....I suspect that Nintendo will quickly kill off the Switch so it does not canabilize Switch 2 sales, assuming they are similar hybrid style system, but that is my own opinion. For all we know they might have 2 systems on the shelves for a while like 3DS/2DS and OG Switch.
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2) a comment regarding your PS1 -> PS2 comparison:
PS1 did have a massive drop when the PS2 launched. It fell by 50% the year the PS2 had its global launch (some 18-19 million to 9.5ish million); Again, the Switch has not yet had such a drop in sales. They have sort of staggered off over the past few years but there is not a 10 million year to year dip yet. The last 2 FY were ~17 mil and ~23 mil. They are projecting 15.5 this FY. That is a gradual decrease.
15.5 million for this FY is not a low number by any means especially for a console as old as it is...it is more than any Fiscal year the PSP or PS3 ever sold,
it has yet to be on the market as long as either PS1/PS2/PSP/PS3,
And it has yet to receive a price drop.
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3) Enthusiasm for Rumours:
All in all, none of these products exist on the store shelf yet. There is no Switch 2 (but we know with certainty there is one close to reveal and coming out), There is no Xbox Handheld (but it seems plausible there is one on the way) and there is no Steam Deck 2 (but the Steam Deck 1 has carved out a nice niche and I am sure Valve will continue to support it, update the hardware so it can perform better given it is targeted towards hardware enthusiasts)
Consider this. The Vita was never really a successful system. We don't know exact numbers, but consensus is it sold at least 10 million units, as much as 17 million (thats wide gap yes)
The Steam Deck is reported to have sold around 4 million units after 2 years of sales (I understand it may be somewhat less or more than this, I understand you can't just walk into a Wal-Mart and get it, but the point remains the same)
The Vita sold about 6 million units ww after 2 years of sales.
Don't let YouTube "influencers" and Internet forums and enthusiast gamer circles distort your perspective and influence your feelings...
The "super popular" Steam Deck is lagging behind the "forgotten failure" Vita, and people feel it or it's successor poses a threat to the current or next Switch...I find that unlikely.
I understand that you are also referencing the unknown - who knows how successful a Steam Deck 2 might be or a Xbox Handheld. They could both blow away predictions. A Game pass handheld might end up being exactly what Microsoft needed to rejuvenate its hardware business. A Steam Deck 2 might finally break into the main stream. We do not know and I do not claim to know the future either.
But, the Switch was successful in large part by its software. The system itself, as cool as it was in 2017, is not going to sell itself. It had Zelda, Mario, Pokemon, Smash Bros etc... basically it had Nintendo games that people wanted to play, and these are games that we can only find on Nintendo systems. It is the same as the under powered Game Boy or DS before it - the software will sell the system.
So I would bet that, at least for the mainstream, if the Switch 2 comes out and is weaker than any Xbox handheld or Steam Deck 2 a future YouTuber will do a comparison of, it will not matter because it will not have the Nintendo games that are attractive to the masses/mainstream. I fully respect that you could end up being right, and your (and others') feelings could end up justified, and you can bookmark this post and say "Remember that fun banter we had a while back? I told you so" in 6 years time, but if I was going to bet on which system in 6 years has more sales, I would not be picking the rumoured Steam Deck 2 or the rumoured Xbox handheld over the Switch 2, even if it doesn't live up to the OG Switch in sales