My three cents again...Popularity, nitrous, heroin, greed, infantile/inconsiderate and violent behavior--mix it up and pour the scene down the drain....
After Touch of Grey/In the Dark come out, the crowds became larger than in previous years. They started playing stadiums again and the scene simply got a lot bigger and included a lot of people who were basically there for the party much more than for the music...."One in 10,000 come for the show".
As I see it, the real problems seemed to begin in Summer of 88 with the remergance of the ol' N2O. There were folks rolling into Alpine with U-Hauls filled with tanks. Once these were unleashed on the scene, lots of other problems got worse and new ones emerged. In Buckeye in 88, I remember seeing people walking around with big balloons and thinking WTF is that all about...tons of people never at shows before (mostly youngsters) there to get as whacked as they could. "Hippie mafia" there to make as much money as possible.
Plus, heroin was making a come back in the early 90s so now you had a whole bunch of newbies who were also junkies running around the lot, selling bunk everything and trying to get whatever they needed regardless of the results of their behavior. In fact, there were a lot of venues the Dead had always played, that they weren't allowed to play anymore (say around 89-91) for various bad reasons. Remember all the fuss about them doing the reunion in Alpine...no more Civic Arena in Pittsburgh, etc.
I don't think you can blame the overall demise of the scene on phish heads (most of them were seeing Phish actually) or necessarlily on "college kids", but moreso on a combo of several things.
Increased popularity after In the Dark, Nitrous Oxide seemingly everywhere, and a ton of new people who were there mainly to get blotto with no regard for the history of the band and the element of family in the scene.
Once completely F'd up, some seemed to decide they had a right to crash gates and act like fools in all sorts of ways.
Not that some of this didn't go on before, but I think there was just a lot more people doing a lot more stupid shit. The scene simply got too big for it to stay the way it had been.
It became a very large and eventually uncontrollable confluene of problems which basically came to a head in Summer 95.
IMO, it was inevitable that what used to a typical lot scene in the 80s was simply not possible anymore with all these changes having happened.
It was just a matter of time before some really bad stuff started to happen---you all know the rest.