This post was what really convinced me: [Pitch] Inherit isolation by default for async functions - #99 by gwendal.roue
The core issue is that concurrent functions are just less-useful than nonisolated synchronous functions. For an asynchronous function, it makes sense because the await is already required. But for a synchronous function, I do not think the author of the function should be able to artificially disallow synchronous execution on behalf of all clients.
Now, I might hear another argument in favor of them that sways me. But I'm pretty sure that callers should be in control here.
(which itself is another good argument for ergonomic means of shifting work to the global executor)