Ah, yeah smartphone and desktop apps don't do js(afaik) - but the web-browser access is still big. If gmail's web interface went down for a day, I think a lot of people would notice.
Gmail doesn't run JavaScript in e-mails, though.
I am not sure - when you post enough js-code into a gmail compose "window" it will crash.
My guess: the WYSIWYG editor uses its own HTML parser and filter, written in JavaScript, that has some O(n^terrible) corner case that your paste test is hitting.
I don't know of any email clients that execute JS inside mails. I don't think we will see such clients anytime soon.