Even if quantum computing turns out to actually be infeasible and classical cryptography is secure for the next millennia, it's still a good feature to have a third independent encryption layer in the protocol. It makes it that much less likely reliant on the other two being bulletproof.

How sure are you? Assign a percentage chance to it and the cost of exposing old messages, and compare that to the cost of this dev effort.

We know governments are using it, and there's likely a lot of sensitive data transmitted through Signal, so the cost of it happening in the next 20 years would still be substantial, so even if the chance of that timeline happening is small, there's still value in investing in forward secrecy.