From what I see in the past couple of decades in popular languages, is there really a justification for breaking changes, from the POV of project maintainers?

Also, I prefer using ' for the thousands separator and was happy when C++ adopted it. It's less visually intrusive, especially with variable width fonts, and some calculators even use apostrophe. Also, in identifiers, the underscore has semantic meaning: foo_bar is different from foobar. But 1'234'567 is meant to be identical to 1234567, so underscore isn't the best choice.

I see the appeal but there are two problems with this: “solvable” is not the same as “easy”, and that similarly also wants fonts which make × more distinct from x. In both cases that's something which is perhaps approachable for dedicated developers but it seems likely to turn newcomers off of the language far more than deliver any real benefit.

> the underscore has semantic meaning: foo_bar is different from foobar. But 1'234'567 is meant to be identical to 1234567, so underscore isn't the best choice.

It's kind of odd to argue that inconsistency counts against use of the underscore but then argue for adding another distinct use for the apostrophe. That would require care for every tool which works with the language from compilers to highlighters, which seems unlikely to be worth the hassle since, unlike mathematical symbols, there's not much precedent for that convention.

$ LC_NUMERIC=de_CH df --block-size=\'1KB

I don't know if there is a locale that uses underscore, I didn't find one when I looked a while back. Perhaps there should be...

    new Set($(".value").map((idx, elem) => elem.textContent))
    Set(8) [ "", ",", ".", "         ", " ", "'", " ", "٬" ]