[acdha]

You rather spectacularly missed the point: I wasn't saying not to do the fancy stuff but rather to start with something which degrades well and then have your JavaScript enhance that basic experience.

If you want to know why this is a good idea, you should start using something like getsentry.com or errorception.com to record your JavaScript errors. That won't tell you who couldn't execute JavaScript at all but it'll show how many times something didn't load due to a flaky ISP, adware, buggy anti-virus, odd browser settings, etc. With progressive enhancement, those people still have a reasonable chance of at least seeing the content on the page. With a pure JS approach, they're only going to see a blank and will probably be heading over to a competitor whose site degrades well.

(Note that this is only the question of the site working at all. In most cases, the progressive site will also render considerably faster – Twitter found an 80% improvement! – since the pure-JS approach breaks the browser's prefetch optimizations and requires much more work to achieve comparable performance)