If you're small, you can get that redundancy on a PaaS. If you're big enough to move off a PaaS, you can rent physical machines in multiple datacentres.
Size is not the only thing that dictates whether one should use a PaaS. It's entirely possible for a tiny generalist team, or even a single developer, to develop an application whose requirements prevent it from running on a PaaS. But that doesn't mean the team or solo developer should have to handle all the hassles of operating physical servers. (Source: I was that solo developer for many years.)
