Phil;
The molds for kits are seldom "retired". Since considerable set-up is required to produce a model, most manufacturers decide on an annual release schedule of a few dozen subjects and concentrate on those molds. Other molds will be typically be used in future release schedules. Molds can also be leased by other companies for release under their own label. Then, there are the blatant knock-offs of popular subjects. So ... kit seldom stay out of production for extremely long periods.
There are a few exceptions, however. Some tools are modified (AMT's Maxwell being re-tooled into "My Mother the Car" and Revell's Lockheed Electra being retooled into a P-3 Orion come to mind) which means the original subject can never be reproduced. Sometimes molds are actually destroyed. There is a rumor that many of Aurora's molds were destroyed in (take your pick of a train wreck, tractor-trailor mishap, barge capsized, etc.). What I am pretty sure happened is that Monogram purchased Aurora's assets, then recycled many of the molds of odd subjects, subjects alien to Monogram's corporate philosophy and that sort of thing. Aurora used high-quality steel, and the recycling recouped some of the purchase costs.
But, some companies will actually go to the effort of re-engineering or reverse engineering lost molds to reproduce a replica of the original, lost subject (can you say Polar LIghts?)
Be patient, and/or be a savy e-Bay shopper. Just about everything you could want will eventually come your way.
'nother Phil (Schenfeld, in this case)