I used to think like this too, but somewhat surprisingly to myself, I have actually come around to deciding the web API is the most user-forward API that closed-source software can target, and it is actually good for end users for closed-source software to target it. This is because web API software is distributed as plaintext to be interpreted by software under user's control, the browser. This gives a ton of power to the user. We as end users can develop browser extensions, CSS hacks, and so on, to make this closed-source software work better for us.

I frequently do this; why can't I paste into this password text box? Screw it, I go into dev tools and enable pasting into it. Who decided grey-on-grey text was a good idea? Screw it, I got into dev tools and make the text black. Why can't I copy the text on this page? Screw it, I go into dev tools and yank it straight from the DOM. Why does this stupid animation take so long? Screw it, I go develop a greasemonkey script to set the animation timer value to 0. This is all quite easy with web API software, but would be be extremely difficult with a compiled binary.

This is to say nothing about the cross-platform benefits. No one's shipping a HaikuOS binary, but most web apps work just fine there, depending on browser feature support.

I think it is also a good choice for open-source software for the same reason. It provides a common, well-tooled API for users to hack the software they are using. But open-source software has other hackable options that make it less of a strong argument there.

Native toolkits definitely have a lot of nice features that the web API doesn't (yet) have, and there's a ton of value in the consistency native toolkits provide. But I think I've come around to valuing hackability over those features, so I'm actually now a web API fan. Again, surprising myself with this switch :)