I don't think the number of people finishing games has changed drastically. I think the ratio of gamers who complete games has declined because of the addition of millions of casual gamers. FPS's and Wii brought a lot of people into gaming who don't have long attention spans when it comes to completing games.
Games in the 90's were pretty easy to beat true, but in the early days games were frustratingly hard. Contra, Battletoads, Ghouls n Ghosts? I know of few who have completed any of those before emulators. No saves made games much tougher.
Depending on how you judge it, a lot of MMO's are not beaten by most. WoW has raids that few guilds on a server have completed. FFXI and FFIV are broken to the point you can't beat them(I guess that wasn't intentional though). If reaching the level cap means completing an MMO(it doesn't imo), then many more people are completing them.
Shorter games may be on the uprise, but luckily they aren't going to stop making epic games. Elder Scrolls V is said to have 120 dungeons. FFXIII-2 is said to be a little smaller then FFXIII(which took an average of 47 hours to just complete the story, and 65 hours to complete every quest).