When development is your livelihood, you target the platforms that make money and only the platforms that make money. That pretty much means Windows and maybe Mac. Anything else is throwing money away.

Sure. As a developer I would've used Qt (or Gtk) because even on the platforms that Electron provides, both provide a much higher quality user experience. You may not care about Linux on non-x86 or QNX or BSDs now, but you've ruled out ever caring enough to consider them.

As a user I loathe Electron apps. They're slow, don't respect native mannerisms (e.g. VS Code for ages didn't put titles on windows you minimized), and horrifically bloated. I try not to use free Electron apps (Discord and Slack get used in a browser), and I would never pay for one. So there's that.

The fact that they eventually put it there tells me that it wasn't a fault of Electron in the first place, but fault of the designers/devs. Electron allows you to do all those nice things you love if you want to, so I don't see how it is Electron's and not designers'/devs' fault.

That's not something a native MacOS application dev would ever have to think about. If Electron requires extra work to make your app behave like a normal app that's absolutely a fault with Electron and means that Electron apps will often present poor the user with a experience.

Developers in the 90s often supported 5 or more different platforms. How is it that modern development is so much worse?

That's an awfully bold, if unbelievable, claim given that cross-platform toolkits have existed for a long time.