Not necessarily. If the current "default" is imperative, then the burden of proof is on the functional advocates, because they're the ones advocating for change.
Burden of proof is very relevant if neither side gave an argument.
People are doing A. Someone says "Do B instead!". "Why should we do B?" "Well, why should you do A?" At the end of that extremely unproductive exchange, what are people going to do, A or B? They're going to keep doing A, because they were already doing that, and nobody gave them any actual reason to change.
So "burden of proof" isn't meant in the sense of this being a formal debate, with rules. It means that, when ahf8Aithaex7Nai said "Prove your own claims before you demand proofs from other people", that ahf8Aithaex7Nai is wrong. OOP is the current default in terms of the bulk of professional programming; if FP advocates want that to change, it's on the FP advocates to provide reasons, not on the OOP advocates to prove the correctness of the status quo.
Right the status quo works. Computers are serving us. The only question of course is whether rigorous adaptation of FP would make them work even better.
The proponents of status quo only need to prove that the existing approach works and is useful. And I think that is proven already by the very existence of status quo. It wouldn't be there if it wasn't somehow useful.