It would be great if there was a setting (much like the "Enable for Field Maps/Collector" toggle) to enforce what a web map is able to be opened in (classic vs new).
We spend a lot of time configuring smart forms etc. only for less experience users to click the classic option (the current default) and have none of the automatic calculations, rules, etc. be enabled. There is probably a use case for the opposite as well, where you want an older web map that was designed to be used in Classic to only be used that way.
Perhaps this would be less of a problem for me if the default was the new map viewer, but I imagine other users want "Classic" to remain the default for now, so I think it would be best to give the map owner some avenue to enforce a user to interact with a map in the environment it was designed for.