I have a lot of thoughts on the whole Express, FFP, last weekends attendance "conundrum".
I don't think it's fair to say that FFPs made up for very little of the attendance as a whole. If you look at opening weekend, Halloween night, and then the "Encore" you can see clear differences in attendance. Every "Multi-Night" ticket is good for Opening Weekend, and it shows in the crowds that have become the norm. Opening Weekend for me, is less fun than pretty much every other weekend because of how crowded it has gotten. Halloween night had the die-hards without commitments there, and while it was a significantly less busy night than those weekends in September it was relatively steady. When you put that final weekend into play though, the event was much more sparse and there's a lot of reasons for that, but HHN was competing with EDC for starters, and then start to factor in the late announcement of the "Encore", and you've got two founding reasons of why it was less busy.
As far as Express as a whole, I think that Express for what it's retailed at is attractive enough to get people to purchase it. There were quite a few nights where Express sold out, and that can not be blamed on Frequent Fear Pass users. I don't think their attendance greatly impacts the Express lines simply because of the investment attached to purchasing one of those tickets.
As far as GAC, I think the only way they were impacting the lines was people having multiple GAC, and using that to breach the system. The way Universal's system is set up helps significantly with reducing the impact of the GAC, and I think the current system of just asking for the tickets would probably be enough to have quelled that problem.
TL;DR HHN "Encore Weekend" will probably come back but with more planning on Universal's part