Whatever it is, it should not come out until 2023.
Don't cut the Switch's lifespan short, or pull support a year before it's replaced like the Wii.
Give it a long and full cycle, like Sony does with its consoles, or Microsoft since the 360.
(They should however do multiple hardware revisions of the original Switch over the next 5 years)
I think comparing the Switch to the likes of fixed consoles does a little bit of disservice.
The Switch will age much more poorly than the Xbox One or Playstation 4, that's just because of Nintendo's hardware decisions.
But if they can keep it around for a long time like the 3DS and sell it at a low price point whilst releasing a more premium device in say... 4 years time, then I am all for it.
I guess that's the thing; the 3DS aged like a banana left in the sun, yet they still gave it 6 years before they replaced it. People don't buy the Switch for its graphical capabilities so it won't matter so much if it falls further behind.