> I would be very surprised if ZGC became the default, because it incurs a significant overhead penalty to eliminate those GC pauses

That throughput penalty is not very high with generational ZGC. It's not zero, but it's not very high, either. What ZGC mostly does is spread the memory management activity more evenly across the duration of the program (this does have a cost due to barriers being active more, but it's not huge). But we have some work planned to improve ZGC even further, which is why I didn't say I think it will become the default imminently, only eventually.

> ZGC performance is _catastrophically_ bad with <=1 cpus

That may well be true, but the JVM can automatically choose a different default algorithm for these circumstances. Indeed, until very recently, the default for low-CPU environments was different (Serial) than for bigger ones (G1).