Well, besides the fact that Nintendo has a history of trying completely new things, they also stick to what they have. Especially in the field of handhelds.
For example, look at the Game Boy series. There have been several generations of Game Boy. Think of the original Game Boy, Game Boy Color Game Boy Advance.
In total, this series (which incidentally were all backwards compatible) ran from 1989 to 2010.
With the successor, the DS a similar story. Introduced in 2004, the DS eventually ran through 2014 with the DS, DS Lite, DSi, and DSi XL. However, its successor, the 3DS, could also run the DS games. The 3DS and variants lasted from 2011 to 2020. So a total of about 16 years.
The Wii, including the Wii U, lasted from 2006 to 2017, with the Wii U unfortunately being a flop. However, this is still 11 years worth of support.
There are more examples, but it also happens enough that if something sells well, Nintendo will continue to do it for several generations. And given the current success of the Switch, it seems to me 100% plausible that we will get a Switch 2 that is backwards compatible with the current platform. When will that be? That could be 2022 but also 2023.
And even then you don’t have to expect a 4k version. I think you should be happy that you can play handheld 1080p and that it will be docked 1080p 60fps. And we’re not talking about the titles developed for the new generation, but more about the current games that will run better.
What new titles will do is hard to say.