Staffing isn't helping but it is indeed Fastpass that is causing it. I went for years when there was no fastpass and it never was like it is now. Even the attractions that still have massive ridership ratios now have lines. It started immediately after the Original Fastpass was initiated and it has only gotten worse. Do more people go now, probably but there are more parks now then there was so the increased attendance should have not been affected. Now with what is called declining attendance, there are still lines in all four parks.
It stands to reason and is scientifically valid that every action has an equal and opposite reaction. If you take two lines and speed up one you are going to slow down the other automatically. There is no mystery to this. The way it has always been set up there is only a few that will get in the fast lane and all the rest will wait for them to be taken care of first. I remember the old lines and they appeared long, but if you joined a line it was first come, first served and it moved almost constantly. There never was a feeling that you were second class, when you got to the front of the line you were the next to experience that attraction. You were not stopped while 10 minutes worth of riders got there late and walked right in front of you. It never really worked, it was a dog and pony show. Because you might have gotten a Fastpass for one attraction you probably did save time. You lost the time that you saved plus more as soon as you had to go to a standby line because they either ran out of Fastpasses or you couldn't afford one. It was always a bad idea for the park guest, but they lead you to believe that you saved all kinds of time and you were just being conned. The time you saved on one ride you more than made up for on the one that you had to standby to see.
I suppose if you decided that you would only see the ones you had a fastpass for then you were fine with that. Personally, when I went to WDW I wanted to see everything that was there and before FP I could. Not anymore. It only takes one CM to point a finger and tell you which stanchion to stand in or where the closest restroom is located.