That's the basic design that rqlite[1] had for its first ~7 years. :-) But rqlite moved to on-disk SQLite, since with WAL mode, and with 'PRAGMA synchronous=OFF' [2], it is about as fast as writing to RAM. Or at least close enough, and I avoid all the limitations that come with :memory: SQLite databases (max size of 2GB being one). I should have just used on-disk mode from the start, but only now know better.
(I'm guessing you may know some of this because rqlite uses the same Raft library [3] as Nomad.)
As for the upgrade issue you mention, yes, it's real. Do you find it in the field much with Nomad? I've managed to introduce new Raft Entry types very infrequently during rqlite's 10-years of development, only once did someone hit it in the field with rqlite. Of course, one way to deal with it is to release a version of one's software first that understands the new types but doesn't ever write the new types. And once that version is fully deployed, upgrade to the version that actually writes new types too. I've never bothered to do this in practise however, and it requires discipline on the part of the end-users too.
[2] This might sound dangerous but in the current design of rqlite, the underlying SQLite database is completely rebuilt from the Raft log on startup (which is fsync'ed on every write). So any corruption of the SQLite database due power loss, etc is moot since the SQLite database is not the authoritative store of data in rqlite.