In the days before the App Store, developers charged much higher prices for most programs (e.g. $50+, not $5), and every year or two they came out with a major new version and the upgrade cost a substantial portion of the original purchase price. For really good apps, that I'm going to use a lot, I have no qualms paying for a subscription for an app. What I don't have time for is the "subscription for subscription's sake because the VC that funded development likes money" that goes on with some apps that are not really good. Ivory qualifies as really good, to me.
The opening of the App Store sparked something of a gold rush, to get apps into the store to sell to an easily reachable market of millions of customers, and then we got a race to the bottom, as some developers decided to compete on price instead of quality, driving away some of the better apps for ones more pedestrian, or even mediocre. And in the process, the public got the mistaken idea that software was only worth $3 or 99 cents, and got angry when prices were any higher.
So... I prefer one-time purchase prices on things rather than subscriptions, but I'll go with a subscription if I find the app especially useful (and in some cases, I'll go for the "lifetime unlock" for really great apps, if one is offered, like with Apollo).