markrichardb: Aye. It’s not uncommon for the first game of a franchise to be charming yet rough, the second a perfectly polished gem with all the greatest ideas, and the third to tread water. Franchises like Half Life and System Shock are cemented further in our memories because they never had that downfall.
This. I think it's a symptom of cycles of testing. First game is the proof of concept -- is it going to succeed? Where did we go wrong, how can we fix it? If First succeeds, a doorway of potential is opened... Second takes all the mistakes and lessons of First, fixes and polishes them. Second is essentially the First, but matured and fully realized, at the height of its power.
If Second also succeeds, Third continues what was good in Second, but probably isn't as ground-breaking -- after all, the devs made all the mistakes with First, succeeded with Second, and want to ride on that wave of success with Third and subsequent games. So Third has potential to become the gateway to milking the market for all it's worth, keeping that door of potential open as long as possible... long after it was worthwhile. If Third won't wrap up the trilogy properly, then you get that downhill slide.
Assassin's Creed is the prime example. Maybe the same happened with Halo, Fallout, Bioshock, and Final Fantasy XIII. Baldur's Gate, Mass Effect and The Witcher ended at the right place (then again I have a good opinion of ME3). Sometimes the trilogy just needs to end, right here, for good.