That's a really big topic but in general I'd combine theming and markup to one language (not necessarily coupling CSS and HTML in one file but having something that does both with similar syntax and rules), make things simpler so there's one clear way of doing something rather than using a generic container for everything, etc.

You'd have to show an example of such a language, it's difficult for me to imagine a good combination between styles and markup - it's good that they are as separate as they are, intertwining them makes the web less useful.

Obviously deprecating a few things will happen over time but the reason web dev is how it is now is because technology used to be a lot more limited and websites were a lot simpler. 25 years ago, nobody knew what the "modern web" would look like so it was made up as people went along. We know what specifications we would need now if anybody went back and re-did them, I think you'd end up with something better.

We know the specifications we need for current development and applications. If we were to "reset" and build new specifications now, how do you know they won't be just as useless in 25 years?

I don't think they're comparable. You won't use a GUI and drag-and-drop for everything obviously, you'd still be able to add sections with code. The fact that Wordpress powers almost half the internet is proof that a simpler web dev experience like this is in demand and it can work. Most websites don't need something complex, just something that supports rapid development and is intuitive, and doesn't make it easy to fall into bad practices. Like I said, it's a hot take, but I would prefer it so much this way.

Why do you think such a tool doesn't exist as of now? The platform definitely enables you to build it if it's possible - and a less capable platform will not make it easier to enable such a thing.