> when most of the functionality of web apps were built and rendered on the back end, with new state being displayed after full page refreshes or simple AJAX ca

> when most of the functionality of web apps were built and rendered on the back end, with new state being displayed after full page refreshes or simple AJAX calls. Most websites (including this one for example) still function that way. It works fine. JS frameworks simply are not required for most of the work the web does nowadays - you can add interactivity and immediate feedback where it makes sense very easily with vanilla js.