I'm not sure that's true. Here's the UI hall of shame, highlighting the worst of the 90s: http://hallofshame.gp.co.at/shame.htm

Most of the stuff there would be just... normal now. It's quite unusual for SPAs to have a decent consistent UX. And the slowness would never have been tolerated back in the day.

the author of that page seems to believe that mouse hover feedback is bad, even when it's a simple highlight?!

I think that's mostly it. Design has a much larger role, and form-oriented development with common controls doesn't cut it. You couldn't imagine an app like Facebook in a forms-style UI, it's almost ludicrous to imagine.

Looked at retrospectively, forms were just one step above green screen applications on a terminal, transplanting one set of structural idioms to another, like for like.

If forms are essentially terminal apps, is FB much different from a teletype news service, with hyper-filtered content and an infinite set of data sources?

I see massive sea change in connectivity and immersiveness of today, but not really in what we're trying to achieve.

2001 was Mad OS X Cocoa. 2020 is JavaScript. The standard for UI was higher then, but was lost to the almighty god of cloud-based web apps.