You realize, there are people who think differently? Some people would argue that if you keep working on problems you don't have but might have, you end up never finishing anything.
It's a matter of striking a balance, and I think you're way on one end of the spectrum. The vast majority of people using Julia aren't building nuclear plants.
Refusing to fix a problem that hasn't appeared yet, but has been/can be foreseen - that's different. I personally wouldn't call it unethical, but I'd consider it a negative.
Literally anybody could forsee that, _if_ something scales to millions of users, there will be issues. Some of the people who forsee that could even fix it. But they might spend their time optimizing for something that will never hit 1000 users.
Also, the problems discussed here are not that things don't work, it's that they get slow and consume too many resources.
So there is certainly an optimal time to fix such problems, which is, yes, OK, _before_ things get _too_ slow and consume _too_ many resources, but is most assuredly _after_ you have a couple of thousand users.