The stable and coherent APIs Google wanted to make all the way back in Android 4.4 was met with strong resistance by power users and data collection companies.
Many API changes are either extremely minor ("set this flag if you want to keep the worse, legacy handling behaviour") or extremely important ("actually ask the user before you drain their battery tracking their location in the background"). The new handling of notifications (requiring explicit permission) is also a godsend.
Google hosts and distributes apps effectively for free (let's be honest, how many Android users actually even pay for apps). The least you can do to keep the app running is to make it work with the modern API once every three years, with an announcement about the API bump requirement a year in advance.
Before Google did this, the Play Store was filled with crap from the Android 2 era that crashed on startup. Paying the one-time $25 developer fee and chucking an APK over the wall isn't exactly a good way to create a decent app store.
You don't have to comply with Google's desires if you don't want to, but you'll have to host the APKs some place else. Termux has had to do this, unfortunately.
Apps that don't get updated can't be easily installed on new devices. They'll still work and be easily installable on old devices (matching the targetSdk version) in case you rely on an Android 9 tablet embedded into your POS.
I think it's entirely fair to expect a yearly "is everything still okay" check from application developers.