I don’t think we do want to (though note this is not about what most people think of as “threadsafe types,” whose instances can be shared—not just copied or moved—across threads). Especially with the cost of uncontended atomic operations coming down, carving out a special category for types less threadsafe than Int doesn’t make sense to me. The principle of concept requirement clustering says that, if we are going to have ActorSendable, these things go together.
I have some serious reservations about actors, but that’s mostly a different story.