IMO, subs didn't ruin the app pricing ecosystem, the app store revolution did.
In the past; there weren't 100k-1m apps available for everyone to consume and there wasn't a lot of competition along with a lot of marketing and ads either. People had to find software on their own. Trials was the key to demo and indie shops could stick with one platform developing high-quality apps for it.
Now, indie shops including companies have to develop for more than one platform (and multiple OS versions per platform) (thus why Electron has been steadily gaining control of the ecosystem on the desktops), throw in some marketing expenses, and fight against copycats thanks to the app store allowing any one to develop an app, regardless of any quality. It can be very easy for someone to just clone the app and make it work barebone.
(I understand some people only want to support the macOS shops producing such native macOS apps, keep in mind that macOS users is <10% of the overall desktop OS market and to this day, it may not be sustainable to keep working on them especially since apps have a saturation point where new features become much longer to develop or there's nothing to add)
Devs now have to complete on price because why would any customers look at 100$ app over a 50$ one and then 30$ app over 50$ app and the so on down to the last dollar. At some point, it became unsustainable. Going back to 30$ isn't an option, casual users today aren't going to look at it. It's easier to eat 1$ per month than 30$ one time.
Not to mention any potential recurring costs they have.
Developers are human beings that need to survive (I'm generalizing it, there are companies that are just evil but that's not really the majority of the app store right now (excluding profits)), that means they need to know if they'll be able to have food in a year or two.
Upgrades are very difficult to do / maintain compared to subscriptions. Keep in mind there are various types of subscriptions; subscriptions should be defaulting to allowing people to keep what they paid for with no features disappearing when they stopped paying. In other words, not like notability where they remove features. If you paid up to this month, all features added prior to this month should be yours to keep forever but not bug fixes nor support.
The reason developers love subscriptions is that they don't need to worry about putting aside new features to save for an upgrade, they don't need to do any prep work for major releases (licensing, marketing, support, etc). Plus subscription means they don't have to maintain any more than one version, and many more.
Upgrades are not coming back (at least in the form we used to know it), it's not sustainable for a lot of developers in this era of multiple devices, multiple platforms and so on.
Asking for upgrades means you're asking the developers to stop adding new features and simply fixes anything broken in the version you're using. They should not be giving you new features, period. Buying a version does not entitle you to future new features. This is the preconception that a lot of folks have that's not really realistic nor sustainable.