I think most developers who used ExtJS will have love/hate memories of the framework. I mostly used the framework from 2007-2014 at a previous job, and at the time the extent of most developers' JavaScript knowledge was how to manipulate the DOM with Prototype, jQuery or a similar library. ExtJS was a whole other animal compared to that with its own Java-inspired class system that sat on top of JS, a JSON-style declarative approach to design UI layouts and an extensive data model, store and proxy system that was actually incredibly powerful.

This day was always going to come, however. Sencha's commercial side has always had a habit of making bad decisions, and have always focused on attracting companies rather than developers. This led to initial success on the enterprise side of things - they had a large percentage of the Fortune 100 on their customer books. The problem was that developers didn't use ExtJS outside of the enterprise setting. This was primarily due to the decision to use GPL for the open source license and later to completely alienate individual developers by introducing a 5-developer minimum purchase for commercial licenses. Although I was highly competent with ExtJS and Sencha Touch, I never used it on any side projects mainly because of these issues. All of this meant that when it came to hiring developers with ExtJS experience, it was always a struggle, and I believe it is for this reason that it never took off outside of the big corporates.