I never really doubted BC. Nintendo is on record ( albeit in a somewhat coy manner) that the current Nintendo account is set up to ease the transition to the Switch successor. Cutting off legacy Switch software seems antithetical to that. Why restart that ecosystem at this point with 140 million ( and going) Switches out there?
Enhancements though?
Question is will it be free enhancements or they'll paywall them ala select PS4-PS5 upgrades. I'm guessing it will similarly be case by case but something like bumping resolution and frames IMHO should just be brute forced via the hardware without paying( not that I claim to know how these things work).