Gnome devs have a clear vision of what Gnome is supposed to be: simplistic, designed for touchpad and keyboard, not mousy-clicky, and staying out of your way.

Nobody questioned this.

People install it, miss stuff they are used to from traditional desktops like Windows or Plasma, and bolt that back on using extensions from third parties.

Like the Extension feature intends it.

They install those extensions from a different source than Gnome itself (Gnome from their distro repos, extensions from the website).

Even those you can install from some distro repos can cause your whole Gnome DE to crash. However this isn’t even the main problem; the point is that it’s able to crash your DE at all. If they did it correctly only the bad extension would crash. If that doesn’t work for some reason, the whole extension layer/API may crashes without taking the DE with it. If something phenomenally bad happens your DE should crash but, as the absolute minimum, your open applications should still keep working so you can save things and restart things gracefully. What you just did is blame the extension devs again.

And then they complain when those third party add-ons from a different source aren’t perfectly integrated or in sync after an update.

It’s about your computer (well, everything graphically) crashing, not some small problems. Get your facts straight.