It is not possible (sorry if i missed how) to lock the app after a certain amount of time, and to have to type a pincode to access it (or fingerprint) in order to further secure the access at the app.
I find this feature very nice in telegram, and would love to see it deployed on deltachat.
to your question: i agree that there are usecases for this feature and this is also discussed in the team from time to time. however, not so long ago, we decided against it.
that said, things are always moving, and there were some efforts in an at-rest encryption, so, might be that by that something will come back (maybe with fewer UI involved ). but there are no concrete plans for the foreseeable future.