I could see him trying them later in his career, especially RVV.
Not too long ago we have had Vincenzo Nibali being very good in MSR. Granted, he was a much better descender than Evenepoel, but otherwise I don't think he was better-suited to the race than Evenepoel.
We have also seen that same Nibali, as well as Valverde and Pogacar showcase that races like RVV can be contested by determined GC men. Alaphilippe himself did so too and he is not that much bigger than Evenepoel (Julian is 173 cm and 62 kg, according to Wikipedia).
The Ronde van Vlaanderen of today is also a bit different from the one before 2012.
Evenepoel, being Belgian, might have a similar motivation that Nibali had towards MSR and give at least a decent set of attempts later on.
That being said, I am not sure what Evenepoel has shown on cobbles so he might just be awful on them.
Roubaix I cannot see him participate in unless it really ends up being a missing piece in his palmares. Or if he fails to replicate his earlier success in future GT's.
If he can show aptitude on cobbles I would not put it beyond him. I cannot see someone being both competitive in Roubaix and in a GT GC during the same season though.