I want to add to this as I think this is a big part. Another part is guild runs. Our guild has been mostly 10 man focused but we always ran 25 man twice a week. With 2 10 man groups who knew the fights there were enough of those 20 (like 12-15) who wanted to 25 man and we could fill it in with others from the guild. We would have about 10 regulars who ran just 25 man and then we would fill the last spots with whomever was on or PUGs.
I raided 5 days a week. With shared lockouts that was gone. I leveled another tank and tried to get 25 man going, but of those 20 in 10 man groups, only a few could/would dedicate time for a second raid ready toon. The 25 man failed!
A lot of the 10 man folks just log on to raid and log off. I rolled a PvP toon for arena, but without PvP there would be no reason to log on between raids. Not everyone likes to both PvP and PVE.
So we have seen people sort of drop off after years of playing.