Accessibility used to be a decent argument (at least to those of us who cared) but it isn't any more. I say that as a co-author of WCAG2.
1a) Screen readers (e.g. JAWS) don't support JavaScript properly 1b) Only very modern screen readers support JavaScript properly (JAWS 10+) 2) Upgrading screen readers is expensive 3) People with disabilities statistically have lower incomes
QED we can't expect disabled users to have JavaScript.
To address the points:
1a was true a long time ago, but modern screen readers do support JavaScript and things like ARIA.
1b is somewhat true but that proves it's a technology rather than an "accessibility" problem, the screen readers weren't implementing JavaScript rather than it being an actually accessibility issue. As such WCAG2 made the decision to treat it as such.
2 and 3 are somewhat true, however many people with disabilities have access to non-profit, state or federal programs that support them with technology. Also free, open source projects like NVDA are increasingly advanced.