And making it so that "many clusters" look exactly like "one cluster" is one of the goals the kcp prototype was exploring (although still early) because I hear this ALL the time:

1. 1 cluster was awesome

2. Many clusters means I rebuild the world

3. I wish there was a way to get the benefits of one cluster across multiples.

Which I believe is a solvable problem and partially what we've been poking at at https://github.com/kcp-dev/kcp (although it's still so early that I don't want to get hopes up).

If you have 2 clusters, wouldn't you just blue/green them for rolling changes?

At a high level, almost anything you would want to use multiple clusters for can be done on a single cluster, using e.g. node pools, affinity, and taints to ensure that workloads only run on the machines you want them to. As a simple example, you can set up a separate node pool for production, and use node affinity and/or taints to ensure that only production workloads can run there.

One exception, as other have mentioned, is blast radius - with a single cluster, a problem with Kubernetes itself could take down everything.