Depends on how old and creaky I am! I want fun, crazy herding dogs (always Aussies, probably a Koolie or Kelpie at some point) forever along with my Tollers. So as long as I can provide them all that they need, that's what I'll have. And I don't think even when I'm old that I wouldn't be able to have these breeds. My current dogs, while active, are easy to live with. So at this point I'd say that if I'm having a hard time with the breeds I like, then it's probably time for me to no longer have dogs.
That said, if I ever go for a smaller dog, it'll be either a random rescue mix or a Border Terrier.