You have a pretty narrow definition of a platform. Consider https://en.m.wikipedia.org/wiki/Computing_platform

Yes, you are right, I didn't consider the checkcast instruction.

Yes, F# does not have this particular limitation, but has few of its own in different places. Most notably, it lacks HKT (arguably, because of CLR awareness of generics).

BTW, alternative approaches to stack-safe recursion do exist in Scala. For example, https://github.com/slamdata/matryoshka