Some of the problem with web UIs is poor discoverability of shortcuts. With some modest effort, you can create a web UI that is as easy to use for an expert as a mainframe form. It won't be exactly the same as a mainframe form (you could do that, but we're out of "modest effort" here), but it would be as easy to use. But there's no good story on how to discover the expert interface. For one thing, nobody even experts there to be an expert mode, so there's a vicious cycle in which it is never developed -> never used -> never developed etc.

But all the capabilities are there to turn a "down" cursor into going to the next form element down, or having CTRL- shortcuts for popular fields, etc. A nontrivial part of the problem is just that people don't even think about expert-mode users, let alone start trying to program for them. People see a web page and they are in a mindset of novice-mode UI, both developers and users.

A Web 1.0 interface with Javascript expert-mode shortcuts, and some selective pre-loading of things like form screens could probably just about match a mainframe UI on its own turf, then be able to extend it with modern touches. And while applying All The CSS could possibly slow it down again, the results could still be at least a bit prettier than a mainframe form.