In many cases I'd say that's not an issue, and JUCE is underrated/underused as a general-purpose UI toolkit.
Please explain!
It also encourages you to adopt a coding style that's consistent with the rest of JUCE, particularly in terms of memory management. This is one of the things that makes it a lot more "framework" than "library", and again might not be for everyone. Take a look at the "Object lifetime and ownership" section in this page:
And even in an absolute sense, you can't confidently say that dual-licensing limits popularity in the long term. If JUCE and Qt were free for everyone, perhaps they wouldn't be able to have the full-time developers they both currently have. So maybe they wouldn't be as good, thereby decreasing popularity against competing dual-licensed libraries. This is all wild speculation, of course, but it's an example to show how their model works.