I think it'll definitely depend on how people will react to the titles. If they speed up development of the titles at the price of quality, then I think fans will notice that the series will not last forever. If sales go down, so will production. The story must have a conclusion though. They have definitely set themselves up for an arcing storyline. Desmond will can't fail like the previous subjects, and he can't go on forever, cause Abstergo is right behind him. But for what we've seen, there will have to be a conclusion to Desmond's story one way or another. Now, after Desmond's adventure, they can easily adapt a new protagonist in the same universe and explore new timelines. But that's where they will most likely kill the series.
If anything, I would like them to innovate the series completely for other timelines. Assassins from every time period couldn't have possibly used the hidden blade for all of their assassinations, right? Even with tradition, there can be a way to influence a more modern age to their creed. Also, how will Desmond dodge bullets? Not every Abstergo agent is going to be carrying a nightstick. I think gunplay would be apropos for the modern era, but only if done right.