It is already possible to kick a device (go to Settings: Security in Element, for instance).

I've filed an issue at https://github.com/matrix-org/synapse/issues/10813 for inactivity timer logouts.

However, my point still stands that fallback keys are a good idea - you should kick the device out deliberately (manually or with a timer) rather than rely on a protocol quirk meaning that E2EE randomly breaks after some period of inactivity(!)