There is a market, and it has players. One way is small, inexpensive standby UPS: Something with fast switchover, but without the capacity to survive anything

There is a market, and it has players. One way is small, inexpensive standby UPS: Something with fast switchover, but without the capacity to survive anything longer than necessary to avoid some flickering and to conduct an orderly shutdown if the badness persists. This is the cheapest way. (And for small low-voltage loads like Raspberry Pis or consumer routers, there are kits available using things like 18650 cells.) Another way is a small-ish online UPS. These have no switchover time, but they're more expensive because the parts are bigger, and they're less efficient than a standby UPS. Another way is a ferroresonant transformer. This isn't cheap, but it is lasting (no moving parts or chemistry to degrade) and can work through some flickering. And finally: Motor-generators, with a flywheel to pick up the slack. These work fine, but they're big, alarmingly expensive, require maintenance sometimes, and there are good reasons to be scared of spinning flywheels.