Honestly, if they never need anything more from the developer, a perpetual license and never spending another dime seems fine. However, in modern times, OS vendors (especially one named after fruit) tend to break a ton of APIs and change rules with every "major release," meaning developers have to invest a ton of effort to at minimum meet all those new requirements every year (!) or else the app will at best look out of place, more likely look totally screwed up and exhibit sudden "bugs" due to the unexpected OS changes, or at worst, crash.
Then users are suddenly all over the developer to provide an update "so I can use this on Tahoe" or whatever, and unless the application is in its honeymoon period where new sales suffice to keep money flowing, the developer is gonna need recurring revenue in order to do recurring development.
Right, but then you're providing tangible value to the customer and thus it's warranted to charge again.
The fairest thing to do is when a customer buys the software, they're entitled to that exact version forever. Or maybe 1 year of updates and bug fixes if you're feeling nice. If they want the next version that supports the next OS, it's fair to charge some more.
This what IntelliJ does. When I buy their IDE I can use it forever, and then they offer discounts for renewing. Pricing seems reasonable even though I'm currently generating $0 from my software development so I keep paying.
> When a customer buys the software, they're entitled to that exact version forever
Sure. And if the audience is very broad that could be perpetually sustaining -- they're each year selling new licenses to people who just became old enough to want that type of software. You can see how a major IDE can afford to do that, right?
What it is really bad for is more niche software. This software often reaches its whole addressable audience in just a few years, and then revenue dips toward zero until the random occurrence of an OS breakage, which may take 1 year or 5 years. And some people even expect that "unbreaking" update to be free.
Obviously everyone has a right to refuse to buy under any business model other than a perpetual license if they wish. But I think that refusal to consider paying for subscription-based software under any circumstances greatly decreases the options that are going to be made at all, as the perpetual license model is absolutely unsustainable to all but the largest developers, and the apps with a very broad, naturally-replenishing audience. Everyone else will either quit or make ad-supported crap.