Any type of UX interaction was always messy and not very scalable using older javascript.
Sure it was do-able and libraries like jquery, which have been shown to be slow, made it convient and easy to write it. But ultimately it was messy looking code.
Sure using the basic built-in UX like a button that post a form or a site that just had images on it was simple. Once you added interactivity and the web became a lot more than just dry course dumped information people wanted a way to navigate that.
Even the most simple website can be improved with a hamburger menu to have an always available nav menu instead of the classic scroll to the top or have a fat footer at the bottom.
So a convoluted CSS solution is available by using a type 'checkbox' and seeing if its check in the place of the hamburger.
But really that's shoving your view logic and your ui logic together.
That covers the state of javascript before these libraries. What hacks, work arounds, or messy spaghetti code can we create to make it work. Since it worked for us 10 years ago then making maintainable, readable and sensible code should be considered bizarre and strange?
I see this push back a lot. I think it is cause originally learning HTML, CSS and Javascript was easy compared to what it is now. You learned mostly HTML and CSS and you learned the basic idea of functions and making API calls to the browser like document.getElementById or console.log.
But deep down I've met a lot of these 'older' and 'experienced' javascript developers that wonder why we are moving in such a 'necessary' direction. I find most of them don't really understand programming itself. What the javascript language is doing. What the concepts are. The fact that a lot of calls are to the browser api that are exposed via javascript and not natively part of the 'language' itself. Sure Node has some similar those functions because it's built-in to their library to mimic the similar environment.
Point is, if you enter any other language like Java, Python, C++, C#, Ruby ect, these way we use to code in Javascript were essentially anti-patterns and extremely round-about ways to solve problems instead of using proven design patterns that have been established throughout other languages.
So in conclusion I think the push back is that right now Javascript has become more difficult to learn or adapt at first with the notion that 'its easy'. But the truth is that it has caught up to more mature languages but the browsers have still been lagging behind requiring extra work to learn how to adapt tool chains to build these so they are compatible.
I have no issue with people writing a simple website in React, Vue or whatever SPA or UX framework they want. I think it's crazy if they just serve the straight javascript files as creating static files of their site is rather easy and makes the initial load times to paint quicker and javascript-free friendly. Therefore you get both worlds. Nice maintainable and readable code that is very easy to modify months later with the same benefits of your typical static pages.
Long rant but hopefully you can better see why things moved in this direction even for simple websites.