I think this is an excellent question, and I think the answer is yes, isolated functions should implicitly be @Sendable. If you form a global actor isolation closure from off the actor and capture non-Sendable state, that closure is not callable today. And actor isolation implying Sendable-ness matches everybody's intuition because it's exactly what we tell folks about actor isolated types.
The only question is how this interacts with region isolation SE-0414. If you form a non-Sendable isolated closure off the actor and capture non-Sendable state, that closure still is callable if the captures are in a disconnected region that can be transferred. I think that's still okay; we could make isolated closures be @Sendable by default unless you have written some other annotation to indicate that the closure is not-Sendable but it may be transferred (e.g. with a disconnected annotation per the future directions of SE-0414).