Woe be to the person still on dialup modem. 300kb on a 56k modem is around a full minute download if memory serves. Of course said modem user is probably already drowning in javascript frameworks anyway.
I hope that 300KB refers to the backend code size, since one of the selling points is organizations run their own version. There's no way you need 300 KB of frontend code to render HTML from a database.
I understand your point, I just hijacked your comment to voice my frustration about current trends.
I tried checking the actual size of the JS being loaded, but since the site for an article I opened made over 500 requests to load the page after I turned off add blockers I gave up. In their case, 300kB _is_ tiny.