I'll be antagonistic and say that the "original gameplay" is a bit of a myth. Everything looks rosy in retrospect, that's nostalgia. It was easy to be original back in the day, because even the most basic things hadn't been done yet (such as your example of online shooters).
Even if you look at "original", "indie" games such as Darwinia (which I loved) or Psychonauts (which I largely loathed), at the end of the day they're still a squad-based tactical RTS and a platformer with some funny dress-up.
Psst:
Portal's gameplay wasn't very original. Ever played
Narbacular Drop? And especially in terms of visual language it was so formulaic and typically Valve it made me groan. I envisioned Gabe Newell kicking around a rusty trashcan while munching on lard burgers the entire time.
That said, yeah, COD6879567 and Tomb Raider348690458659067 are just the publisher milking a dead cow. But really, who plays that stuff any more, other than the bought game reviewers?
There's games where the sequel was better than the original (ground control 2, tf2, wic), there's games where the sequel was worse (gta 3, gta san andreas - both of which I found terrible), but I agree with Tsurayu that sequels alone don't hurt new IP. It's the consumer demand that does, and management's fear of it.
Tight development time budgets hurt original IP much more than anything else.