Been suggesting this for a while - Nintendo kept telling their investors that Switch's lifecycle would be far longer than usual and it would be unlike any of their previous systems. Nintendo also hinted at this possibility in their project NX reveal. I'm surprised so many still expect a relatively conventional break to a successor, rather than, as you say, another iterative upgrade in 2025 (which might be the point where the base model struggles to run, or can't run, new games from Nintendo).
Yeah add in the fact that a good number of people with inside info have heard similar, I'm surprised people are so dismissive of the idea.