Sooo, here's an interesting idea.
We can directly see the dropoff rate between games and sequels now thanks to SteamSpy, can't we?
So you could compare the dropoff between, say, Videogame 1 -> Videogame 2, and GameyGame and GameyGame: The Gamening.
If a game has been particularly bundled it might skew up the math, though, if only by merit of being redeemed by people who wanted other games. Also the amount of time he original game has been released / how many sales it's been a part of could play a major role, so maybe this isn't entirely reliable. At a quick glance, we have:
Binding of Isaac - Owners: 2,533,144 ± 86,178
Binding of Isaac: Rebirth - Owners: 829,183 ± 49,491
Half Minute Hero Super Mega Neo Climax Ultimate Boy - Owners: 379,431 ± 33,512
Half Minute Hero The Second Coming - Owners: 28,593 ± 9,206 (OUCH! Why?)
Avernum Escape From the Pit - Owners: 83,459 ± 15,727
Avernum 2 Crystal Souls - Owners: 7,877 ± 5,832 (3 years gap, but still, this makes me sad)
Deponia - Owners: 392,568 ± 34,086
Chaos on Deponia - Owners: 136,781 ± 20,131
Goodbye Deponia - Owners: 82,687 ± 15,654
Trine - Owners: 1,203,205 ± 59,568
Trine 2 - Owners: 2,104,256 ± 78,619 (Whoa, almost DOUBLE the original? It's odd to see games step out of daddy's shadow)
Torchlight - Owners: 1,384,806 ± 63,880
Torchlight II - Owners: 2,764,975 ± 89,988 (...or maybe just adding proper multiplayer functionality sells more copies)
Anyone else wanna look up some notable dropoff / rising sequel numbers?