Sustainment and the OEM

Now that Original Equipment Manufacturers sales of new product are waning, they are diving head first into trying to rebuild their ability to sustain legacy equipment. They obviously need to do this to drive revenue and survive the existing market conditions.

With this new push, and the realization that the OEM may not have been the actual original manufacturer of a specific part, the question arises:

"What are the pros and cons in directly engaging with third party players for sourcing spare parts instead of an Original Equipment manufacturer?"