This is an interesting perspective, and gave me something to think about!

I don't think the Steam Deck is quite there in terms of adoption to justify an across the board tax. The order of operations is kind of reversed, where Steam is reinvesting money made from previous sales towards R&D and Hardware ambitions, rather than using the Steam Deck to bring in users. But if you're developer that benefits from the Steam Deck's existence, or saw a sales bump from Steam Deck sales, or some other benefit like that, I agree it's a pretty good trade-off in that case.

Nintendo is a bit different because they sort of focus on their own thing and everyone else is secondary. Something like 80% of software sales for Nintendo platforms are first party, so it's mostly a Nintendo machine. Frankly, I think they should take less of a cut. Indies do really well on Nintendo though. They have a kind of pseudo-monopoly of a younger casual gamer demographic, and they maintain that user base by putting out great software. It is an interesting counterpoint though.