> There are few things less accessible than web applications depending on bleeding edge javascript that might not get successfully executed. There are few things more accessible than text in an HTML web page on an HTML web site.

It's not that simple, you can create highly conformant SPAs if you want, there's just more issues you have to watch out for. And even the best semantic HTML and ARIA will need Javascript for things like content replacement - for example, there are valid accessibility reasons why no one uses form submission for table filtering and sorting anymore. JS is also required for implementing and keyboard interactions for WAI patterns, not something you can get out of the box with plain HTML.

Ultimately, the determining factor for a11y is not your choice of front-end technology, but rather whether you're willing to make it a priority. Janky SPAs sites have not made it one, and I don't think an MPA is going to fix that. Raising the bar on quality is really what's needed.