Redundancy is great, absolutely, but there are multiple ways to do it and this is not necessarily the right one.
Which is better: 3 DCs for 2 DCs worth of traffic, running the same stack but isolated failure domains... or 4 DCs for 2 DCs worth of traffic, that work in completely different ways, where 2 are left cold until needed. I can see some advantages of the latter, but not a lot.
This also isn't about fault tolerance at the code level like in, say, avionics. In those scenarios having multiple distinct implementations makes sense, but that's very different, you run all paths at the same time and check they agree etc.
Which is better: 3 DCs for 2 DCs worth of traffic, running the same stack but isolated failure domains... or 4 DCs for 2 DCs worth of traffic, that work in completely different ways, where 2 are left cold until needed. I can see some advantages of the latter, but not a lot.
This also isn't about fault tolerance at the code level like in, say, avionics. In those scenarios having multiple distinct implementations makes sense, but that's very different, you run all paths at the same time and check they agree etc.