I wouldn't worry about it too much, honestly. Plastics in general have a lifespan of "After you." Unless you're putting them through pretty heavy use and impacts there should be no breakage, and if that part is anything like the rest of the Bionicle Pieces I know, that rubber is there to stop general wear on the piece more than it is to stop it going completely limp. The majority of friction on the piece should be coming from the "clips" that hold a ball joint.
Thermoplastic Polyurethane should have a similar lifespan to regular Poly, with the difference being as long as you don't subject it to extreme heat. Best I can find is you're looking at 10-15 years, same as most other stuff. Though that's for general applications. I imagine molded pieces will be longer lasting.