I don't even feel confident in that claim. If keeping memory usage under 10 MB requires a significant amount of developer effort, that is effort that isn't going into something else that I might value a lot more than, say, 20 MB of RAM.

Not just developer effort, but also sometimes program runtime efficiency. RAM is cheap, RAM is pretty fast (compared to disk or network); cache shit there when it makes sense to do so for overall efficiency.

10MB is a fairly ridiculous modern standard to limit yourself to, IMO (and I started on the C64 and generally do dislike and try to avoid bloat).