They can always make the Switch 2 completely backwards compatible to sweeten the deal. Being able to play the previous lineup right off the bat is always a big plus, and I'm pretty sure the technology for it is not that complicated, especially if they still use the same cartridge format.

Super New Switch Pro X Deluxe U

No doubt a new Switch with updated power is coming at some point.  I would think it won't be branded as a Switch Pro as Nintendo would likely discontinue the current Switch and run with the Switch "Pro" and handheld only switch variant when that happens. I would expect it to be significantly more powerful than the current switch and to come out in 2020 or 2021. I see the switch concept lasting quite a long time with several updates and new variants released. I just don't see what Nintendo would do hardware wise if it wasn't continuing down this same path.  It would be nice if they came out with a console only version that was similar in size to the Apple TV. This version could have more power as you don't have to cram it all into a handheld form factor you could likely run in hotter. 

Assuming Nintendo sticks with mobile chips for let's say another 8 years what will the landscape be by that time for a traditional console?  The cloud will likely have reasonably high adoption rates by this time and there are diminishing returns on power.  I just don't see Nintendo going back to a traditional console ever again. If and when the Switch concept loses momentum who knows what Nintendo will do at this time. 

A switch needs to play nintendo exclusives and some lesser powerhouses ported from other consoles + indies.
Therefore it already has enough power and a switchpro would be a bit nonlogical because it would firstly even then not get games like monster hunter world(playerbase can not be divided on the same console) and secondly i do not think a big portion of switch consumers cares about small graphical updates for the already existing games.

I think they need a real handheld, actually.

Currently the Switch is already "Pro". They need something to fill the handheld gap left by the DS/3DS. A smaller system, with more of a handheld feel.

I bet when switch starts slowing down, they will revitalize it with a 'pro' and 'mini' versions. Nintendo decided going long with their sucessful hardwares.
Better chip technology in both, one scaling up the specs, other keep the same but in a smaller design and focusing in saving battery.
Release in 2020, 2021 for 3 more years of switch.

Then in 2023/2024 a switch sucessor

The thing is though, if they make Switch 2 backwards compatible, they lose their chance to rip off consumers by making every second Switch 2 release just a rehashed full-price Switch 1 game at a higher res. I can see them axing back compat for this reason alone.

Xavier is an alternative. Should offer 50% more performance over the Tegra x2 with lowered clocks, reduced voltages and aggressive binning...
In bandwidth bound scenario's (I.E. 1080P docked mode) the X2 and Xavier would provide a massive increase in performance over the X1 chip.

Isn't Tegra Xavier designed for cars though, not for mobile devices?

From what I've read the thing is big and sucks 30 watts, far too much for a portable system.

Yeah it is. But the same held true to the Tegra X1 (Tegra Drive CX/PX) and X2 (Tegra Drive PX2).
Tegra can scale upwards and downwards fairly well in power consumption... Tegra X1 and Tegra X2 were 20w parts in the "drive' configuration, where-as Xavier is 30w... And Tegra X1 was still forced to scale downwards for the Switch anyway.

But... In saying that, it is a massive chip, so it would likely benefit from being fabbed at 7nm which will solve the bulk of issues anyway... But the real benefit would be the big uptick in bandwidth plus a more modern and efficient GPU architecture.