There's a lot here, to be honest these things always come back to investment cost and ROI compared to everything else that could be worked on.

Java 8 is still really popular, probably the most popular single version. It's not just servers in context, but also Android where Java 8 is the highest safe target, it's not clear what decade we'll be in when VarHandle would be safe to use there at all.

VarHandle was Java 9 but MemorySegment was Java 17. And the rest of FFM is only in 25 which is fully bleeding edge.

Protobuf may realistically try to move off of sun.misc.unsafe without the performance regressions in a way that is without adopting MemorySegment to avoid the versioning problem, but it takes significant and careful engineering time.

That said it's always possible to have waterfall of preferred implementations based on what's supported, it's just always an implementation/verification costs.