I can see Linux on the smartphone becoming about as popular als Linux on desktop. Appreciated and used by relatively few but enthusiastic people.

Think longer. Apple Inc. was founded 44 years ago. They were a PC company until 11 years ago. Can we be confident that in 2031 the libre phone will have gained no market share to the iPhone? What about in 2064?

If the UX does not vastly improve even beyond what is currently good by Linux standards I don’t see a reason for most people to switch.

It might not be the thing driving people to switch, but I'd be hesitant to claim that Desktop Linux is any worse than the competition. Like, I don't have the exposure to comment on Apple's products, but Windows is no better than XFCE, GNOME, MATE, or KDE these days, Android is a toy, and ChromeOS isn't better. Like... I know this is weak praise; Windows versions after 7 got worse as they fragmented (have they re-merged the control panels yet?), Android was born a mobile OS, and ChromeOS was always meant to be cut-down, but the result is that the playing field is level.

I don't know who the CEO of JP Morgan is, but I imagine he might be one of the few who would pull out a Pinephone with a his own custom software stack with encryption for all his communications and perhaps some other special tools.

You're imagining wrong

What a wonderful argument. Really shows your intellect.