The biggest tension between software and one-off purchases is related to bugfixing, and especially security. It makes sense that you don't expect new features for a product that you paid for once. But, what is the case with bugs? If the product mostly does what it promised to do, but sometimes crashes, do you expect those crashes to be fixed for your version, or not? What it you're using a 10-year old version and a new critical (say, unauthenticated remote execution) vulnerability is found in it? Do you expect to get it resolved as part of the price you originally paid, or would you be ok with being told you have to buy a new version if you want this ?