Is it possible to destroy the rupa heavens? This may sound like an unusual question, but - is it possible in theory to destroy the rupa heavens - not by avoiding rebirth there, but for other sentient beings?

Not the tusita heaven - which I understand (correctly or not ha) is part of kamadhatu and strictly one of the jhanas :)

Thanks !
There should be an observer experience any plane of existence. Once the observer is no more, in a relative sense they cease to exist.

All these are experienced through consciousness. When consciousness cease to exist so does all the places of existence.