Database encryption isn't comparable to the keyboard drama. Protecting against malware in your keyboard can be done by using a different meyboard and is of course out of scope.

But if my phone gets taken and an exploit is used to get root access on it, I don't want the messages to be readable and there's nothing I can do about it. It's not like I can just use a different storage backend.

It's also a very simple solution - just let me set an encryption password. It's not an open-ended problem like protecting from malware running on the device when you're using it.

If someone has root access to your apparently unencrypted phone, then they can just launch the Signal app directly and it'll decrypt the database for them.

Which is to say this is an incoherent security boundary: you're not encrypting your phone's storage in a meaningful way, but planning to rely on entering a pin number every time you launch Signal to secure it? (Which in turn is also not secure because a pin is not secure without hardware able to enforce lock outs and tamper resistance...which in this scenario you just indicated have been bypassed).