I surf with JavaScript disabled by default and selectively enabled for a few frequently used sites that directly benefit from it. I rarely find sites that are unreadable without JS--certainly less often than I used to experience sites that were unreadable because of it. On the rare occasions I find a site that wont work without JS (most common symptom:completely blank page) my decision more often than not is to close the tab and move on with my life. I don't think I'm missing out on much and my computer's fans no longer scream constantly when my machine is idle with the usual dozens of open tabs.
I've run into several sites that have weird issues without Javascript, but they seem to always be things that could have been implemented with traditional markup: missing form components, misplaced images, things like that. I'd say about 50 percent of the time[1], however, media-focused sites with complicated image-viewing "galleries" or a more obscure video player, are totally useless without it.
It can be frustrating to have to go through this process of navigating to a site, realizing I've broken it, and then reloading with all the crap turned back on, but yeah, like you said, it's better than having my CPU revved up just to have those "SIGN UP FOR OUR NEWSLETTERS!" modals flying around the screen.
That's because the designers of most web pages want the features they provide. When you say "optional" you don't actually mean optional, you mean removed. Such a thing already exists, it's called Gopher[1].
Most websites today don't even gracefully degrade. One of the trendy blog/article sites that gets posted here regularly (it might be Medium) is just a column of text with lots of whitespace, but the text is loaded via AJAX, so without JS, you can't even read it.
HA! That would actually be lovely. Something easily-discovered, like a "isactualhypertextnotaturingtarpit" attribute for the <meta> tag (OK, maybe something less verbose) would probably solve half the problem.
