I wasn't so much blaming G+. I saw the same thing in the days of FP+. In the days of FP-, the parks were busy past midnight. But then under FP+, once people ran out of fast passes, they were less eager to stay. It changed a little when 4th FP came along, but many people didn't book a 4th FP. So the parks aren't as crowded at night, and that = further cuts.
I'm partly speaking for myself, but if I'm in HS, and all the good G+ passes are gone, and the (current posted) wait for Aliens SS is 45 minutes, MMRR down, MF:SR 40, Sinky 70, RSM 60, ToT 70. none of those waits are very appealing to me. What am I going to do, watch Muppets 10 times until park close?
As I also just posted, shorter park hours also give the illusion that WDW is more crowded than it is. People can't spread out time-wise.