Cartridge life really only 3 years??

I have a 27 year old Dynavector Ruby 23 that is fine, and have to say your dealer is just giving advice he has heard, somewhere. I would say if you are so impressed with the sound, why search for greener grass?
Keep your cart.
A few cartridges had suspension material that quickly degraded or dried up, but I think those days are over.
Most cartridge bodies last for a very long time. Far more than 3 years.
My advice: If it ain't broke, don't fix it.

I basically agree with Elizabeth.The suspension could deteriorate.Its sort of a rubber like material.Usually if it was exposed exposed to extreme heat,or something else to cause damage,then that would be different.This could happen to a brand new one also.But basically I have not heard of a shelf life standard time set for them.I imagine there are exceptions,like a company that tried a new material that didn't last as long as others.If it performs normal,I would use it without worrying.There are 30 plus year old ones, selling for high dollars yet,giving their new owners great satisfaction.

It is at least theoretically possible for rubber/elastomer used in the suspension of the cartridge to degrade with age, either from exposure to pollutants in the air (ozone), or because some of the compounds may be slightly volatile. This is the theory, though three years seems exceptionally short a shelf-life.

But, I haven't experience this myself, and I have heard many very old cartridges that sounded very good -- at least this is the case with the better brands.

If it ain't broke - replace it right away; because it might break at any moment.