That's true for the publisher's worries, if they're a one trick pony like Blizzard!, but not the console companies. Imagine WoW came out on XB360, and 50% of owners bought it and played it all the time to the exclusion of everything else. MS wouldn't get any revenues from sales of other games. Imagine Halo 3 is played so much with so much amazing replayability, the 8 million buyers of that game don't buy any other titles for 2 years. What if Assassin's Creed keeps expanding with user content, and 5 million owners don't buy any other Ubisoft games as they're still enjoying AC? The games companies want us to get tired in order to buy more games. What if SingStar on PS3 is so perfect there's no room for a sequel (okay, bad example as Sony will be happy to keep selling new content to existing users!).The first title that pops into my head is World of Warcraft. The popularity and replayability of that game means (in my mind) that Blizzard will "never" release a new game (i.e. a hell of a long time). But the game itself is different from their other hit genre, RTS. There's a split fanbase where the RTS is the only interest (Read: Starcraft 2).
So I think the replayability issue you describe is generally a problem if the new game "fails to deliver" by comparison. If the next title sparks no interest, being vastly different, replayability of the older title shouldn't matter. For instance, if Warcraft III was not so replayable, it does not mean I'll pick up WoW because I have no interest in online RPGs.
Perhaps the human nature of getting bored of stuff quickly is enough to ensure this never become a big problem?
