In the olden days software used to be sold by individual major versions. You paid for version 9, you paid for version 10. Or you skipped versions you didn’t need. You could use versions side by side. The newest installed would import its data from the older ones. etc.
App stores have made this very awkward or almost impossible. There’s no concept of separating major versions. You’d have to buy and install completely different apps to be able to pay for them separately and to use them side by side, but if they’re separate apps they can’t import your data from each other. Not to mention that people seem to hate having “too many apps” for some reason.
Software subscriptions switch the “support per major version” to “support per time of use”. It’s obviously shittier but it’s more realistic than a one-time price and expecting to use the app in all future versions in perpetuity. The one time price would have to be very large to be realistic.