As Graham has already mentioned in this thread, there is a mechanism for customers on Subscription to request earlier versions.
Apart from sounding like the business equivalent of a platitude, it does not work because of the limiting factor of "the past three previous versions" is imposed. Commercial houses periodically need to open up scenes from 5+ years ago, tweak them and have them render out with the exact look as the original. One does not simply open it in a newer version where Mental Ray works differently. In this way alone it is clear that Autodesk's business model is incongruent with the needs of its customers.
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