It's not even an issue with adding JS, they could build it entirely in React and not have it crawl along like new Reddit does. The issue is 'redesigns' which real purpose is to add extra layers of user engagement, ads and tracking.
You learn this pretty quickly in practice, I think every junior dev has tried to make an abstraction the first time they saw some complicated business logic and failed miserably. Now if I think an abstraction is easy I'll do it the second time around, otherwise I'll wait until its absolute (n)ecessary.