The Sulzers have interested me for a while, and while working for MPI I was able to talk with some of the "old-timers" who had worked on and designed them. I've written an article that, if I'm ever satisfied with, will be published at some point...
But to answer your question, the SP Sulzers appear to have been taken out of service in the mid-1980's. I don't know how long they sat down in Oakland, but you are correct in that they lasted into the 2001, or maybe even 2003 time frame.
It should be noted that in addition to v8interceptor's comment about the reluctance of the SP to introduce another engine-line to support, there was also the unfamiliarity of the shop crews who were tasked with maintenance and repairs. Flying in parts from Europe got to be really expensive! In addition, there were lube oil and coolant cooling problems, growing pains from introducing hydraulic-driven cooling fans, and, above all, the problems with the tight-tolerances in the engines which led to broken cranks and other issues. While the 12 and 16-cylinder (the 16's were used on the UP and ATSF SD45's) Sulzers had flex issues, the smaller six-cylinder Sulzer used on the M-K demonstrator actually did quite well and saw thousands of hours in service. It may have actually been a successful design for additional repowers had EMD not released its GP15-1's around the same time. The GP15's had much better fuel economy in idle and the lower throttle notches, which was a strong selling point.
Sorry to digress from a simple question of when the SP retired them; I enjoy talking about M-K's early work too much to pass up the opportunity!