Dixi188
Manufacturers are required to provide product support for many years after aircraft go out of production. Typically 25 years although this may be longer if there are still customers for their services.
I believe that the manufacturer can surrender his manufacturing and/or repair certificate at any time and thus his responsibility for upholding airworthiness (product support) in the eyes of the regulations.
The incentive to keep the original cerificate by the manufactuer is the profit from spare parts sales and repair programs. As design and testing is discontinued from the manufacturing cycle then the attractiveness of after market sales of used but not abused parts take hold.