Bad input crashes app, monolith fails over, other instance crashes. Full outage.
Assuming proper vertical separation, this risk can be reduced by microservices.
Care to enlighten me? Your comment reads like "assuming the best case scenario for X and worst case scenario for Y, X can reduce the risk". Well, you don't say.
There'll always be critical microservices that keep your app running. It doesn't matter if all your other services are running if the one serving up core functionality goes down.
If your engineering rigor is so poor that you can't get reliable failovers with a monolith, god help you keeping microservices running.
