Nintendo will milk this hardware even if it's underpowered compared to 'competitors'. I know Nintendo doesn't necessarily try to compete with Sony/Microsoft and does their own thing, but they do of course need the hardware to remain somewhat capable if they're going to keep getting third party support for newer games. I'd expect this to dwindle by the Switch is five years old and it won't be possible to get certain third party games anymore (unless company's continue to make PS4/XBO versions of their game for more than 1-2 years). Nintendo has to hope that because there is probably going to be 70m+ Switch consoles sold by the end of this financial year, third party developers will want to at least try to make a Switch version or even make some exclusive Switch titles.
I fully expect the Nintendo Switch in its current iteration to have a 7-8 year lifespan. It will be more like the Wii & 3/DS lines as even when there is talk of a new console, I think this would be supported continually and die a very slow death. If you buy a Switch this year you'll definitely get three years out of it IMO, so don't worry about that. Depending on the games you like, in that time we'll see more Pokémon, Zelda Breath of the Wild 2, Metroid Prime 4, Bayonetta 3, more Mario, maybe even a Mario Kart 9, and plenty of other quality first party titles.