So my problem with this is that it presumes that is a bad thing. For you and your friends it may well be but the rather harsh truth is because people enjoy this game so many different ways not everyone plays one world for the same amount of time. Differing playstyles simply do not allow for it.
There is nothing wrong with someone finding enjoyment in starting a new world every week rushing to get endless resources as fast as they can and that's it. At the same time there's nothing wrong with wanting that to take longer. There is something wrong with wanting others to be made to do any one playstyle if they don't want to. The FTB team handing out heavy-handed configs for everyone would be far more detrimental than not.
The problem with modded minecraft is that it is not suited to be played with the same people again and again. It's not football(either one), dota, LoL, or any other game with pre-established meaning and rules. Similar people choose to play it because they know what to expect. With minecraft people do not need similar reasoning to choose to play and thus it is difficult to establish an environment in which people looking for different things can play together.
The FTB does have a few packs that presume on certain playstyle traits; these are not the most successful packs. They cater to specific groups through the playstyle they lean toward the most. Packs like RR and Big Dig do this as well. They simply will never be the number 1 shining star of packs akin to ultimate or unleashed. Those two packs offer a relative area in which a much more generalized group of playstyles can thrive. It can be played slowly or rushed and quit or rushed and build or anything in between. So while the big packs may not work for everyone or work to have every playstyle in it at once; the ones that don't fit are corner cases and there is generally another pack somewhere that will fit that corner case. Even if there isn't the new FTB launcher is being designed to allow those to create areas in which they do. It is also making it easier to allow others access to these areas.
All in all if you want to play together with a group of people you'll want to find packs that work toward specific playstyles akin to RR, MF2, UHS2, etc. and find people that understand what is in those packs and then play together.
I never said there was anything wrong with other playstyles.
Notice though that the OP has seen 'lots' of people quitting. Those same people would pick unleashed over unhinged, but they quit, Id argue, because unleashed overloaded them on the ability to gather resources. Unhinged was not as popular not just because of slower resource gathering, it also has mostly older mods in it, and not that many of them.
Id offer some evidence to the argument though, beyond just the "people can do what they want" defense. A friend of mine played Final Fantasy online, and he told me that as the game lost popularity the game designers started making things that were previously very hard to get very easy. He said everyone had super strong items, the best rare item drops, etc. And... they had a surge of players at first, because they enjoyed having the things that they couldnt get before that, but then they lost players like they had before. He said people said that even though they had more items it ended up being more boring to play. BUT they ALL loved it at first. Unlike the Final Fantasy game, the players can dictate, kind of, what the game plays like. PEople demand Quarryplus... and we get the ridiculous Quarryplus. (IMO) So the evidence im giving here is that other games that do trying giving players what they want 'ALL TEH THINGS!' don't end up being very successful at it.
Ultimate was a super popular pack because... (looks around carefully and whispers) ... Gregtech... created a pack unlike unleashed, where people had more than just 'a building game', they had a resource gathering game EVEN weeks into a world, if not a survival game anymore (MPS kills that).
Again, nothing wrong with enjoying the building aspect of the game more, but as the OP said... lots of people quit because, I think, they want MORE than a building game.