I dono about 15-20... 5 hours take months and months to put together. The hours and manpower of a decently funded film
IMO people seem to expect too much from FPS's these days. They're not something that allows for more and more maps and missions to be tossed on like RTS or old school games. *shrug*
It takes a lot to put together what is essentially three full length movies in a row that are beautiful, intelligent and above all else - interactive