(I didn't catch the original mention of BetterSlack a few days ago, so ironically, your "Welp, gotta withdraw this!" message here is the first I've heard of it. This seems to be my usual timing with such things, though...)
As someone who messes with native apps: this really isn't true, at least in my experience. Native apps tend to follow platform paradigms, which usually make them reasonably well designed and structured–sometimes more so than web applications. Usually adding functionality is simply a matter of finding the class that manages the component, rather than digging thorough a bunch of broken CSS (for IE 6 support) and minified goop.
I haven't been messing much with Windows executables for quite a while, but back in the day, I'd "improve" many programs by just editing their resources.