If the developer isn't providing upgrades then they shouldn't keep getting paid for a product someone already bought. If the argument is that developers need _regular_ pay days then we have dedicated financial instruments for smoothing an irregular cash flow.

In any case, subscriptions aren't directly equivalent to upgrade pricing even when an upgrade is provided every subscription period because in an upgrade pricing model the end user can always choose to stop paying and continue to use software they already bought.

Yes, most models converge in the case of frequent updates by the developer and regular usage by the customer and the customer upgrading regularly. The difference is, that when there are no regular updates, why should a developer get constantly paid? And sometimes, there are huge updates, which are not really reflected in value by a reasonable subscription. On the other side, the subscription model holds the customer hostage, the customer cannot decide to use an older version a bit longer and update later.