I don't think that perspective is fair to modern browser engines. The additions of flexbox and grid layouts took them beyond just typesetting, I think (not to mention all of the work on JS engines, providing APIs for a whole bunch more device functionality, etc.)
So it's true that there's still a "typesetting engine from the 80s" in there, but there's also a powerful layer of app functionality built-in as well. It's reasonable to question whether all of this belongs together, but all of this evolution _has_ allowed for whole new classes of applications to be delivered securely and quickly to all sorts of devices in the browser.
As a thought experiment, imagine that we would have started with Lotus 1-2-3/Excel type spreadsheets instead of formatted text. I believe that we would develop tons of layers of abstraction on top of this too, in order to make cells and functions to emulate UI framework. It would probably work as well, and had some weird quirks related to the cell-based nature of this super-powerful engine. Yet, the question remains - would it be a good foundation for modern UI apps?
To me, most of the issues with web apps stem from the fact that foundation is just not built for that task. Like these weird selection issues, where you move cursor 1 pixel more to the bottom and it selects the whole visible area instead of text - that doesn't even make sense. Or jumping page and blocks all over the page while it loads fonts and stylesheets. Sure, we build more hacks on top of existing hacks to mitigate this, but that's just duct taping with complexity, not the engineering.
Right. It's been discussed before, but the web browser is one of the greatest examples of "write once, run anywhere" in terms of the markup that is HTML. If the evolution had happened differently, maybe we'd be all be using Java applets or Flash applications instead. I believe HTML/browser is superior, but I wonder where other platforms would have evolved to.
So it's true that there's still a "typesetting engine from the 80s" in there, but there's also a powerful layer of app functionality built-in as well. It's reasonable to question whether all of this belongs together, but all of this evolution _has_ allowed for whole new classes of applications to be delivered securely and quickly to all sorts of devices in the browser.