you're right, "acceptable" is subjective. but it doesn't seem unreasonable that if you are using an outdated version of a program, it will not perform optimally. to that end, managing both pushState and hashchange adds overhead and complexity. personally, i don't think enhancing old browsers is worth said overhead.

that said, if it was part of a framework i was using anyway, overhead and complexity are much less of an issue, so the question becomes "why not".