In my opinion you shouldn't expect future updates of any kind from software that you own. I wouldn't expect them to keep updating it because it obviously makes them no money. I would expect them to release a new product to keep making money.
What do you mean by this exactly? There are always new customers to sell to, so of course you can make money by updating your product.
Will the buyer continue to receive free software updates forever? No, probably not. Eventually they may have to pay an upgrade fee. But the great part about upgrade fees compared to subscription is that the choice and timing of whether or not to upgrade is in the buyer's hands. Whereas with a subscription, it's a forced update, with the timing determined by the software developer, not the buyer. You pay yearly, or the software stops working now.