There are design decisions that are wrong. Invisible emergency exit signs are a wrong design decision. Making the emergency off button the same color and shape as the light switch is also a wrong design decision. And yes, I think the original problem is also a wrong design decision.
Design isn't just art, design of functional objects needs to be functional. That means that there is an objective criterion for wrong design: if it isn't functional, it's wrong.
It's a design decision if both sides have competing benefits and you choose one. In this case it's a planning / engineering decision: other work is prioritised higher than implementing the previews.
I'm not sure why you say that. Bugzilla had a partial patch with some outstanding issues needing polish. Nobody rejected it, but nobody is working on it anymore. You're welcome to take https://github.com/Dudemanguy/gtk/pull/1 and continue the work in a gtk PR on gitlab.
You can jump in and implement it. The last few comments are basically "here's a patch that works", "ok, open a PR then" (and no follow up)
