Apple does not have to restrict itself to cat names and they do not have to drop the OSX (as in OS 10) name which is already very strong brand. One can better understand the versioning scheme if the "ten-dot" is omitted from the version number; meaning that the current version for example could very well be pronounced "OSX 4.9" or "OS 10 version 4.9", whichever makes more sense.
So, first of all -- as we're in 4.9 currently, there's no reason whatsoever that Apple could not be continuing Tiger support with 4.10 or 4.25 version, as the version number is not decimal. It's telling us major and minor version of OSX (or OS 10).
Then, another thing, whenever Leopard is out, it will be fifth version of OSX (as in OS 10 version 5.0). There's absotutely no reason why Apple should discontinue OSX at some point, the "OS 10" is their brand for a new generation operating system. As it is a very strong brand name, Apple will likely use it as long as they can really offer "the next big thing" that would put OSX to shame.
Historically, as we all know, things have looked like this:
1) System 1-6 (1984 - 1991)
2) System 7-9 (1991 - 2001)
3) OSX (1999 - current)
One could however predict that something major is going to be happening after OSX 6 in 2008(ish), but remember that history cannot be used to predict the future! Apple may have aces on their sleeves, or not. There's nothing inherently wrong with OSX that would need to be fixed with a major revamp; HOWEVER, there are two things that I can see that would justify calling OSX 6 or seven a "new generation" operating system:
A) feature called "running Windows applications on Apple OS"
B) decision of "running Apple OS on non-apple computers"
Should both of them be happening, we would see a direct battle between Apple and Microsoft. It is likely not gonna happen with Leopard, though, because Apple marketing is probably not ready for it. Yet. We'll see...