Mozilla's memory project a while ago showed that there's often a lot of low hanging fruit if it hasn't been a big focus. V8 uses a lot of memory in comparison in my experience. I think this should be generally without a downside.

Couldn't node simply tell V8 to use the earlier defaults if it doesn't have its own tunings already?

According to tests it doesn't. The heap memory optimization will only trigger on mobile devices <512MB ram. The zone memory improvements shouldn't have a negative effect.

There is no downside to keeping the overall memory arena size down: better cache locality, fewer stop the world collections. Node programmers have been asking for this behavior for years.