The VC-25Bs (748s) in progress were already completed and flight-tested - but mothballed near Palmdale because the purchasing airline went bankrupt before delivery. So the existence (or not) of the main production line is not a factor. The US went with the "off-the-shelf" airframes to save some money.
Given that the VC-25As have been in service for 30 years (upgraded as needed), the Bs may not be "obsolete" until at least 2055 or so.