If "pro" is just supposed to be a mid-generation upgrade, not a true successor, then abandoning Nvidia is inconceivable, yes. You'd need an incredible power jump over Tegra to "emulate" it and it would also be a ton of work in terms of new architecture for again, a mere mid-generation upgrade. And if you're suggesting that they will fully move to OG Switch-incompatible software in 2020 or 2021 I don't know how to say how absurd that is. Console gens wind down when sales slow and not a moment sooner, Switch is on the rise this year and there isn't much reason to think that next-gen static boxes are going to suddenly kill it.
As mentioned there's plenty of untapped power in the current Tegras with how underclocked the Switch is. If they just stick that chip in a roomy box with a heat sink they can raise the resolution of existing games to at least 1440p. It won't look that good aside next-gen or even the pro versions of current gen, but it doesn't need to, because Switch has built a large software library and there will be enthusiasts that want their Switch library to look the best it can on a TV, as well as cheapskates or the uninformed that are going to balk at the $500-600 of true next gen and go for something cheaper that can play new games in "4K."
