Certainly it's possible to build redundant systems properly. But it's expensive. All the well-built redundant systems you listed understand that and budget for

Certainly it's possible to build redundant systems properly. But it's expensive. All the well-built redundant systems you listed understand that and budget for it. Most half-baked redundant systems I've seen are a result of "I want four nines, but I only want it to cost 20% more than a two or three nines solution" type thinking.