I've always wondered with these games or at least games like these...
How many GTAs do people want to have in the end?
Once HL 3 releases will be content or immediately cry for HL 4?
I'll make it brief and keep it at these 2 examples but still, what marks the end for fans of game series releases?
(devs not making more does not count, I'm talking about the people saying "when GTA 7" right after 6 releases)
As long as there's an improvement (a significant one), there'll always be demand for more GTA since it's basically a sandbox game. These games no longer take a recycled assets approach so improvement is almost always great. For every feature taken, 4-5 more are added.
As for the hunger for more...people will always want more....and more...and more. It is natural to want to hoard as much as possible, regardless of how much logical sense it makes. No matter if it's the H-game, classic triple-A, indie unicorn...
The end of the line is only when you see an actual decay and no chance of improvement. A lot of EA and Ubisoft trademarks are actually good examples of such. A lot of Battlefield, Mass Effect or Assassin's Creed fans gave up on new titles because they were the decay-ridden filth in their eyes (BF series is especially bad).