This has been discussed frequently:
2015: Proposal: Sealed protocols
2018: Sealed protocols
2024: [Pitch] Opaque Conformance for Protocols
2025: How best to emulate protocols with private requirements?
(and likely more!)
I think if you want to further the discussion, we need more than a vague "wouldn't it be nice". We (mostly) all agree it would be nice, what's missing is the work on an actual proposal and implementation 