And yet in the nearly thirty years since he said that, the browser has come to dominate how people interact with UIs. Both in terms of sheer popularity (not only do we have complex applications on the web, but we also package complex applications with browsers as desktop applications) as well as influence (SwiftUI, Flutter, and Jetpack Compose all borrow heavily from frameworks developed primarily for the web).
I think a lot of this has to do with the idea that Kay is criticising here: HTML as a mechanism for building applications has been so successful because it needs to be interpreted by a browser. That provides a tremendous level of sandboxing that makes it a lot easier to trust whatever website you're loading than the application you're installing. It also provides a grab-bag of useful controls and elements, while still being completely flexible with creating new ones without needing to resort to OOP inheritance chains or raw graphics drawing APIs. This is all to say nothing of accessibility, which is practically built into the browser.
I think a lot of this has to do with the idea that Kay is criticising here: HTML as a mechanism for building applications has been so successful because it needs to be interpreted by a browser. That provides a tremendous level of sandboxing that makes it a lot easier to trust whatever website you're loading than the application you're installing. It also provides a grab-bag of useful controls and elements, while still being completely flexible with creating new ones without needing to resort to OOP inheritance chains or raw graphics drawing APIs. This is all to say nothing of accessibility, which is practically built into the browser.