What is the lifespan of a level D simulator?
As long as it's maintained, and replacement parts are available, there is no limit. Although maintenance could end up costing more than the simulator itself:

CAE says the Series 5000 will offer a 25% reduction in lifecycle costs, which tend to be two to three times the original cost of the simulator during a 20-year lifetime (...)
— flightglobal.com, 2008

The 20-year lifetime mentioned above is what would a training center expect, as that's typically how long an airliner model stays in service in sufficient numbers to promise a continued demand.
Delta Flight Museum still operates a full-motion 737-200 simulator (Delta introduced that model in 1983, almost 40 years ago).