Obviously the architect, and consequently the project owner. The project manager has no say in these decisions, why would they get blamed? I’ve never seen that happen, while I’ve seen the PO get blamed multiple times.

I really think you’ve never worked in a corporation, because what you’re saying just doesn’t happen. The PM gets blamed if the project falls behind, but not if the technical decisions of the architect make problems. A “project manager” is not a manager in the classical sense…