I have no dog in this race as far as Claude is concerned, but this is pushing a false dichotomy. Not using, say, WinForms or something, because it's too limiting or because you don't want to make a unique UI for every platform, doesn't have to mean strapping an entire web browser to your frontend, there are plenty of other options.
The reason frameworks like Electron are popular is that we've spent a long time hammering a square peg into a round hole and there are now a whole bunch of tools for designing on top of web technologies and a lot of designers with experience with those tools. And of course, the fact that code can be reused between the web app and the desktop app helps too. But it does have a performance cost. The fact that you can have poorly performing and bloated native UIs too doesn't change that no matter how well-optimised your HTML+CSS+JS is, you can create something of the same complexity that is faster and leaner using native widgets. And when people opt for the desktop app instead of web app, they typically want something that performs better than the web app.