It used to work back when the software market was growing exponentially. You could take care of old customers using revenue from new customers, because you always had more new customers than old ones. Upgrade cycles also used to be much shorter, and nobody cared about security bugs. So you didn't really have to maintain old versions for long.

It was a Ponzi scheme of sorts, and it worked for a while. Obviously it can't go on forever, especially when growth slows down.

Maybe if you could convince your customers to use a subscription-based add-on service on top of the one-time purchase, you might make the business model keep working for a while longer.