There is the potential to use homomorphic encryption so that encrypted text can support operations like string search while encrypted, so unencrypted indexes would never need to be stored on user devices. It is a huge hassle though - it requires a ton of compute and is still very slow and limited, it's much more complex, and research is still ongoing regarding security. However if you want to truly minimize the amount of unencrypted data on your device this could one day be an option.

The whole thread is about permanently storing every single conversation forever vs conversations that get deleted shortly after. If the latter is chosen, the blast radius is significantly smaller.

Not really, if you accept the risk of it happening but you shrink the blast radius.