Xbox One X's GPU may have been of the same line but it was still a new and more modern design well ahead of the 2013 base model, and doesn't use on-chip ESRAM like the base and S models so it was significantly different, in addition to the new RAM and the addition of HDR and 4K upscaling on both the S and X models.

Switch Pro will likely be of the same Tegra/ARM chip line as well in order to maintain full compatibility, and the addition of tensor cores is much like X's addition of GDDR5 RAM. 

I agree with you, we will only know when there is more information clarifying the situation of the supposed "Switch Pro".
Whether "should" or "should not" count on Switch sales? As I said, imho, this is defined by "content", if the "switch Pro" has Exclusives then it should not be counted.
However ... it is almost certain that the factor that will determine this will be "Nintendo's word" .... If they decide to say that it is "Switch Pro", and not "switch 2", then most of the community and sites that follow sales will consider so, regardless of having exclusives that only work on "New Switch".

You will not be able to duck tape Tensor Cores on an aged X1 chip design. Everything has to be new.

Nintendo Co. plans to adopt an upgraded Nvidia Corp. chip with better graphics and processing for a new Switch model planned for the year-end shopping season... The new Switch iteration will support Nvidia’s Deep Learning Super Sampling, or DLSSThe U.S. company’s new chipset will also bring a better CPU and increased memory.

https://www.bloomberg.com/news/articles/2021-03-23/nintendo-to-use-new-nvidia-graphics-chip-in-2021-switch-upgrade

I like the idea with software, if the Switch Pro gets a higher number of exclusive games, it should be considered a new device.

New 3DS and DSi got exclusive games. Their numbers are counted towards their respective families and they're not considered new generations.

Ultimately, it will depend on how Nintendo positions it, whether they treat it as a successor or a midgen refresh a la New 3DS, Xbox One X, or Gameboy Colour.

I agree with you, in the end it will be what reflects the official word of Nintendo. No objective criteria will be respected, just one thing, Nintendo's official word. If they want to treat +10 years of consoles as a "single Gen", and launch exclusives for one or more "revisions" of these consoles, so it will be, as it was with the Gameboy.

Imho, this is a big bullshit, and unfair, when it comes to comparing "number of consoles sold" and "bragging rights" between fans ... but this is how it works, nothing to do about it.

Ultimately it is pretty much arbitrary really; Wii and Gamecube are more similar in terms of internal hardware than Xbox One and Xbox One X for example, yet the former is considered a generational divide and the latter is not. The lines between what constitutes a new generation and what doesn't are already pretty blurred and inconsistent, and have been for a long time.

Yeah, I think so.

I point to the Wii and DS lifetime sells. Wii sold 100+ mill, DS sold 150+ mill. Okay, so people bought the DS and the Wii for different reasons, but now with a central console like the Switch, I imagine that it can hit 150+ mill handily. I would also point to the Switch pro+ as being another 3 year life enhancer. Perhaps maybe even other form factors for switch. I just don't see Nintendo dropping the Switch brand for a Switch U any time soon.

DLSS would be a long term investment because you need support on software engine level as well and it will take time for games to come out. There is no point to bring a Switch Pro with DLSS for less than 5 years life span support. Late 2021 would be 4.5 years after the launch of the Switch which is in Nintendo's range for a new main device. Maybe they did start developing two years ago and are now surprised how well the Switch is still selling so they have to rebrand it as a Pro rather than a new main device.

DLSS will almost certainly be carried over into Switch's actual successor, just as HDR support introduced with Xbox One S rolled over into Xbox Series S/X. Like you said, a long term investment.

Partially agree with you. yes, the determining factor is extremely arbitrary, in this case Nintendo that says "this is the new generation" or "this is just an upgrade"!
Regarding Game Cube and Wii. The difference between them is clear as the day! As I said, it's not a question of hardware and "power", it is software and content that makes a new generation!
Do you have a new proposal for content between "One S" and "X1", something that is only for X1? no? same generation!
Do you have a new content proposal between "GC" and "Wii", something only "Wii" will deliver? yea? new generation!

But in any case, this is just a view that follows a logical and objective criterion ... which is absolutely useless here, given that the decision about "new generation" and "current generation" is absolutely arbitrary.