Edit: going through the rest of your post. What I mean by 'slow start' is that it'll take a long time for the sales to really pick up. I think it won't be doing great numbers in 2021 and maybe not 2022 either. Pre-orders don't matter, if we're going by launch sales then by all means the Wii U should have been a success. How many people flock to the console at launch is irrelevant, I don't think the PS5 will hit the ground running with general audiences.
Personally, I'd say the Switch will stop at 125 million, 130 at best. Exactly what you think the PS4 will do.
You are not seriusly comparing Wii U pre orders to PS5. No, Wii U pre orders were nothing special, outside of being "sold out" in some stores, which doesn't mean a shit.
PS5 is confirmed to have bigger pre orders than PS4 which was sold out everywhere, you see the difference there? 2021 may be better or lower than PS4 2014, not gonna bet on that but it's definitively gonna be close, PS4 sold 14 million in 2014 and if PS5 sells a bit less than that i would not call this a low start.
The COVID boost is still a thing and will continue in 2021, you must keep that in mind.
What i really find funny about your post is that you didn't even replied to a single point of what i said for Switch, literally just "i think it'll sell 125/130 max". I would be ok with your opinion if you at least arugue, if you look at my post you can see why 125/130 million for Switch is unrealistic, and it doesn't matter how big this number looks to you or to anyone, there's nothing that suggest Switch to sell less than that, but there are a lot of reasons to think Switch will become the biggest console of all time, let alone beat PS4.
While your only argument is "we can't know what's gonna happen with Switch because is the first console like this".
2020 predictions: NSW 25m, PS5 6m, XSX 4.5m, PS4 9m, XB1 3m
In December 2019 i predicted 21m Switch, but that was before COVID, obvius increased my predictions.