Flash was on approximately 30% of websites before. It's hard to understand the argument that it was unpopular amongst devs.

Devs hated Flash for all of the reasons cited above, but browsers didn't support anything better for many use cases.

Indeed. As an example for a long time you had to have Flash to play a video on your page. The HTML5 video support eventually ended the need for Flash to do this, but it came much later and very slowly became supported cross-browser.