Xbox One actually had pretty high 2014 sales in UK with over 1 million units sold. Also Xbox division is actually in way weaker position when it comes to marketshare/mindshare than it was when Xbox One launched (X360 dominated the late gen of last generation in UK). So it's actually opposite situation. Sony launching PS5 with higher mindshare than they did had with PS4 and for MS the opposite. Brexit is good point though but almost impossible to predict.
Historically almost every single console peaked in its third full FY (so this FY for Switch). Then fourth FY they might almost match those sales but after that sales start to slow down more. Of course individual markets might differ like UK where PS4 peaked already in 2014. Also during late gen price drops don't increase the sales anymore to peak levels They just keep sales from dropping even more than they would otherwise. Even if Sony had launched super Slim at 199€ this FY they would had not shipped same amount of consoles as in 2016 when PS4 was 299-349€. I mean never say never but it's almost sure thing that at least globally Switch will already sell less in 2021 than it does this year.
To the first part - i think MS momentum right now is actually comparable if not better than during the 360--> One transition.
Hardcore players already felt neglected during the last 2 years because MS went full Kinect/Casual and Sony won alot of favors because of games like TLOU and the whole PS+ game library thing. Similar to how MS is gaining alot of favor right now because of GamePass, Crossbuy/play, xCloud, full BC etc. They are def. in a better position than they were last generation - just based on price, competitive hardware and marketing.
To your second point - Switch isnt you standard console either. Fact is the standard Switch has yet to receive a price drop nearly 3 years in - this is also the system that will continue to get Nintendo full support. They are also flexible in terms of how and when they can introduce a more powerful Switch - the same way we saw it with the n3DS, PS4Pro or the One X.
By 2021 i expect Nintendo to have 3 Switch models on the market between Lite(very cheap, entry level), standard and Pro system.
Nintendo has usually the most price sensitive userbase - getting the Switch price down even more will absolutely have a strong impact on what they are able to sell in coming years. The Switch is selling well - but its actually expensive for a Nintendo console. Lite is doing well - but its expensive for a Nintendo handheld.
I think the overall market based on the target audience would react stronger to a very cheap Nintendo system compared to a cheap(er) Playstation - for the male core gamer audience a 50 buck price drop 6 years in wont make or break a purchase and more than one PS4 per household is unlikely. 50 bucks on a system for the Kids or a second Switch for the family household can be a big deal and seems like something Nintendo is willing to push.
Next year and beyond will be interesting - people expect the Switch sales to suddenly take a nosedive....i dont think we have seen the full sales potential of the plattform yet.
