The life of the MBZ tranny depends on how well it is serviced (fluid and filter changes) and how it is used. Lots of high throttle shifts will 'use up' some clutch life and gear train life of course. However FAR MORE transmission die from neglect--the fluid is not changed regularly enough and the protective additives get exhausted and the oil oxidizes, becoming acidic and forms gum and varnish, attacks clutch linings, thrust washers,etc.

Unless you really work it HARD (lots of trailer towing, high throttle shifts) or overheat it, IF you change the fluid regularly and keep the cooling system in good condition, the transmission WILL last well over 300,000 miles.

These transmission SELDOM fail from normal wear. 99.9% of the time its due to inadequate servicing. 'Recomended' service intervals (in your case --NEVER) is not adequate for long transmission life--that service protocol MAY get you past the end of warranty--just--which seems to be the current focus of the New Chrysler/Daimler company.