I was the creator of all phpBB styles for RocketTheme (with huge help of few other folks) so I feel like I can add few bits
First of all I still think phpBB 3.1.x is absolutely great forum script !
Really. Template events are not such a big deal. They are useful but I don't feel like missing few ones in our regular release cycle would have a huge impact on our compatibility.
What I think had a huge impact for our club was a fact that phpBB 3.1 was released soooo late. Like somebody mentioned, 3.1 was released after 5 long years. You realize what 5 years means i IT world ? It's like forever
Still I think It wasn't the only cause. I think that bulletin boards world changed A LOT in last few years. Like you guys also mentioned earlier: Facebook, Twitter it all gained in popularity, so user needs changed a little. Still I think it's a temporary. For me it looks like forums are looking for it's new place and certainly has a future. People already are starting to be tired of bloated facebook and it's limited tools soo.. Who knows..
phpBB 3.1 with it's symfony and twig was a huge step forward. From developer perspective it couldn't be better. In short amount of time we added scss, less compiling to phpBB 3.1 and it was easy. We had to hack many things in 3.0.x branch (i know you complained for that a lot). in 3.1 No more. We loved it.
I feel a little bit sad myself that we decided to discontinue phpBB3 premium styles but still I believe for phpBB 3.1 brighter days. It just needs some time
I will try to help to make it happen by contributing as much as i can into whole phpBB community
Thanks for everything !