When it comes to Xbox One X, as far as I'm concerned all you can look at for comparison is PS4 Pro, which takes 20% of PS4 sales. So if Xbox One X does the same then it's hardly going to help MS pick up sales. I can't imagine at £450 it's going to even account for 20% of Xbox sales, but we'll see.
Like any new hardware it'll start well and likely tale away pretty quickly, especially once people who aren't as clued up realise that it's just a 4K Xbox One.
I really don't think MS helped itself with Xbox One S, as it was quite deceptive with the 4K marketing, so maybe people won't realise the consoles are too much different.
As for Switch, I think it would be doing even better if it was stocked up, although stock seems to be all around the UK now and it's no longer selling out, so maybe the Switch sales are slowing a little now too.
PS4 has a few FIFA Hardware bundles and the Destiny one, so I think it'll keep selling for now. There is even a PS4 Pro FIFA bundle.