Yes, percusski, plastic is the bane of durability - except when it is sacrificial and cheap to replace, and just so long as Sonor continues to remanufacture and not just keep it on their shelves.
Ten to twenty years is about the useful life expectancy of plastic, before it starts to embrittle. Buying and keeping replacements won't really help. Unfortunately you can't store plastic because it continues to age whether in use or not. Attempts at sealing out the atmosphere seem to encourage other and worse chemical responses. I haven't heard of any good way to store the stuff, except in landfills...
Rubber you can protect better (I think) with treatments and protection from air and sunlight, and it would be wise to have as replacement set of rubber for your kit. I don't have one, but
you should.