I see exclaves as a significant but intermediate step. Apple is making XNU less of a liability, but they're still playing defense instead of fully embracing a microkernel architecture.
If I had to bet, exclaves will be a bridge to something bigger, either a more modular OS (like Fuchsia) or a CHERI-inspired security model where memory safety is enforced at the hardware level.
Apple is leading the pack in consumer OS security, but exclaves are a patchwork improvement rather than a total rethinking of system design. That said, this is probably the biggest security shift in mainstream OS design in the last decade, and it will take years before we see its full impact.