A lease has a time limit. A lock does not. Clearing stale locks manually is a PITA. I still assume, being a Web-scale contract, the lock would be automatically cleared if the browser is restarted or something. But honestly a lease makes users do better design from the get-go.
It looks like the only potential for a "stale lock" is if somehow the async function passed to the request method hangs forever. But in web contexts I think that would be extremely unlikely for everyday use cases (e.g. most of the time I could imagine the async callback making remote calls using fetch, but normally that fetch has its own timeout). In contexts where it could happen, I'd argue it's better to make the caller explicitly handle that case (e.g. by using `steal`) than potentially leave things in an indeterminate state because a lease timeout expired.