It's also important to consider the developers risk tolerance as well. It's all fine and dandy that the project manager is okay with the risk but what if none of the developers are? Or one senior dev is okay with it but the 3 who actually work the on-call queue are not?
I don't get paid extra for after hours incidents (usually we just trade time), so it's well within my purview on when to take on extra risk. Obviously, this is not ideal, but I don't make the on-call rules and my ability to change them is not a factor.