Do MC Cartridges Deteriorate over time

The good news for you is that you have no way of knowing how it might have sounded 15 years ago.  So, if it sounds good to you now, there is no use in worrying.  And as others have implied, there are expectations that cartridges deteriorate even in storage, but the sound quality one can often derive from even vintage cartridges calls that axiom into question.  

I own several cartridges mainly MC some with 40 years of life (Nakamichi MC 1000 / Ortofon MC30) and have no problems, as explained to me a cartridge repairer is sufficient when they are not used to close them in their own packaging and not to let light take over.
But there are more sensitive mc cartridges than others about the dumper that could deteriorate; a dumper that is very durable and lasts for decades is the Monster Cable Genesis practically (almost) eternal while for the dumper of the cartridges mm they are very perishable worsening the tracing and the sound quality.