Sorry but my experience with Qt has been neutral to negative. I can go and write a textwall about all the issues Qt has and personal experiences i had with it myself, but they're not really relevant here. Your message is basically "people stick with Qt because people stick with Qt so that is the safe bet" (aka "nobody was fired for buying IBM"). Which sure, it is true and a big reason why you do not see other toolkits much, but this doesn't really have to do with what i wrote.
Grandparent's point is that, in something with less functionality, you will hit missing feature X quickly, and it won't be the same missing feature as any ten random other developers. It is just your interpretation that it is, somehow, primarily about inertia or image. Joel on Software has an essay about (missing) features: https://www.joelonsoftware.com/2001/03/23/strategy-letter-iv...
I get that point and i disagree because it makes two assumptions:
1. That you need to replicate Qt to have the same or equivalent features for a GUI toolkit (remember that what i made explicitly that Qt is more than just a GUI toolkit) or you will not have the same features at all
2. That you will "hit" said missing feature
None of these have to do anything with making a GUI library though. #1 is especially questionable since even if you need the non-GUI functionality Qt provides, it might be something the language already has (remember that this is about other languages aside from C and C++) and/or you may even find a separate library that provides such functionality.
And yes, inertia and image does affect A LOT. People do choose projects only because of the perceived safety of those projects from being used by other people. This is why regardless of features, many people stick or flock to languages/libraries/frameworks/etc for their communities and "community" or "ecosystem" is a common concern about something new. This is something you can see often in Hacker News posts too.