I’m not a PM, but I can actually see, how decisions are made. You actually wrote it yourself without realizing: a PM signs it off. He has to take the risk. If it seems too risky, he won’t accept it.

Just go to your PM next meeting and propose to rewrite everything in, say, Rust. Will he agree? Probably not. Did he make a technical decision? Probably not.

Don’t kid yourself with technological grandeur. If your PM doesn’t want to do X, X won’t happen.