Most of the best bass players have stuck to one bass (make/model) for most of their playing career. When they make a switch, it is mostly to move on to another instrument that will be THE instrument from the moment on.
I have to disagree pretty strongly with this statement. There are so many counterexamples, regardless of who you consider "best." Just to throw out some names-
-Flea has used a different bass on practically every album, sometimes within albums (Spector, MM, Wal, Vintage Fender, Signature Modulus).
-Claypool while mostly playing Carl Thompsons, plays a wide variety of them with different numbers of strings and scale lengths. In fact, he purposely used the fretless 6 for a whole record right after he got it, just to shake things up.
-Entwistle- had a huge collection of basses, with wildly different attributes.
- Geddy Lee- J's and Rics (and maybe Wal's)
-Pino Palladino- has a signature P, but did the last Who tour with a Jag.
- Will Lee has an arsenal of all kinds of stuff, and though he is most known for Sadowsky, can be seen playing all kinds of other stuff.
- Chris Wood plays a Beatle Bass (or maybe a copy), upright, and a Cliff Bordwell.
- Mike Watt mostly gibsons, but Firebirds and EBs at various times.
Personally, I mix it up quite a bit, depending on my mood. My two main axes are a fretted 6 and a hotrod J. I also have a fretless 6 and a P copy. Granted, the two 6s are identical but for the frets (but that was more of a fluke than anything), but each bass has its own strengths and weaknesses, and its moment. I don't think my playing really suffers at all for switching among them, although adapting to the different string spacings takes a minute or two when you first switch.