I have already seen you hating about this topic on twitter - are you really open for a mature discussion or do you seek some drama on the internet?
I am not working for canonical but one of the maintainers of the Yaru theme, also a Java Developer IRL so maybe I can share my objections on this topic:
Developing in Java is very popular in the industry and many rock solid big business applications are running with Java. Also Android apps are written in java (or Kotlin, a super language of Java). My operating system of choice for developing Java applications/apps in my Jobs and for private music creation is Ubuntu. So I often were at a point were it felt very natural to think about creating a GTK app for Ubuntu. If you have been working in tools like Eclipse, Intellij or Vscode, deep diving into GTK, its documentation and IDE feels like falling back to stone age. Did you ever try to do this yourself? Even if you can apply a good design with CSS on GTK and gnome-shell it is so far away from Android app development, spring boot server development or JavaFX client development that it really scares developers away. The gnome community is really cool and amazing stuff is developed by so few people and really works really good (also the new shell design is super cool) but looking for alternatives is always a good thing in the technical world (also in day life btw) and it is really important to get young developers to develop applications for Ubuntu that are not familiar with the whole GNOME ecosystem. This is also the reason why most of the “third party apps” you can use on Ubuntu, like microsoft apps, dropbox apps, google apps (there were some in the past) are not made with gtk.
If the Linux desktop should have any relevance in the future were most likely apple and their new ARM based systems will play a big role then something mightier than gtk is a very good alternative. Just try developing an app in flutter yourself, it is really easy, fast and looks awesome.