It would be a major oversight and missed opportunity if the successor was not backwards compatible. Whereas I already have the games I want to play or collect on Switch, it would be an absolute travesty for a new Nintendo ecosystem adopter to not have the massive library of the Switch, available to them at launch.
That said, I have started over with every other Ninty system (with the exception of the GameCube/Wii), so the lack would not deter myself from a day 1 purchase.