Some people just get development. I had one close person who was able to learn webdev and become and highly skilled freelance developer in 0.5 to 1.5 year (having played with computers his whole life), I had another friend who after 3 to 4 years still struggles to do a simple loop, let alone grasp a recursive function, at the same time he makes great designs and has good skills to implement them frontend wise. Different minds, different talents.
That's really the main reason React got popular. Somehow it lets the people who do not know what they are doing get by. At the expense of the users.
Mind you in my case the designer just really does JSX templating with it and styling. The wiring, plumbing, state and so on is not her business.
The reason I use React is mostly its composable, simple templating abstraction and the fact that it works both on the server and the client.
UX is generally improved. If a site/app is done well, it will work w/o JS enabled, loads fast and transitions even faster.
JS typically makes things better, not worse. What makes sites slow is putting in external and/or heavyweight stuff: ads, analytics, tracking, fonts, images and so on.
Most of this comes down to getting past the fear of coding/programming. And it is a powerful enabler, even reshapes thinking. She recently came up to me with saying “why should I write this in Word? We have markdown and I bet there is a tool where I can style this and generate a PDF”.