IMO, it did pan out. XUL and XBL provided the inspiration for much of what's come to real Web standards over the last decade or so. Thanks to XUL, we had a web-like model for front end dev that over time was usurped by a real Web model for front end dev. If we can re-implement Firefox using real Web standards, XUL will have been be a big part of the reason that became possible.
That's great for FF and Mozilla, but it doesn't feel like a win for those of us who were interested in XUL/XBL as a platform for building our own apps, independent of those Mozilla ships. I put a little bit of time into trying to build stuff on top of XUL before giving up on it, and that time ended up being more or less wasted -- it's not like there was an easy glidepath to take stuff built in XUL and move it to Web standards, because back then Web standards weren't up to the task. Combine that with the platform itself being a bit of a moving target as MozSuite/FF evolved and the end result was work I basically had to throw out.
I'm not mad at Mozilla for all that, lots of platforms are complicated to build on, especially ones that are new and still evolving (which XUL definitely was, at the time). It's just not an experience that I feel like I can put into the "win" category. It was a dead end, at least for my needs.
Yeah. I hear ya. As a platform for app development outside of Mozilla projects and products, it has definitely been a bumpy and often roadblocked path.
And Web Components owe a clear legacy to XBL2 (an abandoned W3C draft, the immediate forerunner to Web Components), which obviously had clear links to XBL (though was backwards incompatible).
