Engineers who design coal mining ore chutes coat the metal chutes with plastic to protect them from abrasion. We could investigate doing the same to cane tips. Both using a replaceable plastic boot or applying a plastic coating would increase the life of a cane tip. Of course, coating the cane tip with plastic would dampen its frequency response. The rubber of the cane tip probably does harden with exposure to the environment.
Some rubbers keep their original properties better than others.
UV radiation in the presence of smog, for example, causes most rubbers to become more brittle. Solvents and acids in street gutter water also weaken the rubber in the tip. Personally, I have not noticed the rubber in my cane tips becoming brittle. I also have not noticed a change in the frequency response of the cane due to a change in the rubber. An apoxy-based glue holds the carbon fiber cane together.