Just to be clear, I didn't say there were things that couldn't be done through web interfaces, just that they apparently can't be done nearly as well. This is entirely based on my experiences with both web-based and native application UIs. In the best web-based UIs I've seen, the problem is usually that they're annoyingly slow and/or laggy. Most web-based UIs have additional issues on top of that (formatting problems, etc.) All web-based UIs share other really annoying problems, like when transient network errors happen right after you push a button or enter some text.
They all also have the problem that they have to operate within a browser window, which severely constrains UI design.