Disagreed. If it requires a server side element, it incurs an ongoing cost and a subscription can be justified. And to clarify, by “requires”, I’m referring to the functionality, not having it shoveled in. And the price should be realistic.
Some apps do this well, Sleep for Android is an example that comes to mind. Free with ads, ad-free is an inexpensive one time purchase. You can also purchase additional plugin apps that add functionality that isn’t required or even useful for most people. And finally, they have a cloud plugin app to let you backup your data, you can pay for their cloud subscription which is $2.99 a year, but you can also just use other cloud for storage like Google drive.