I'm not in the business of selling apps, but I enjoy writing books. The best way to sell books, is to build a mailing list of fans so that you can reach them when you have another book ready. It is not about reaching people who do not want to receive your comms. It is about having a direct and reliable connection to people who actually want to engage with you.
There is no easy way to do that in the App Store. It is important that people own their own platform, that includes being able to know who your customers are.
I'm not saying to spam people who purchased stuff from you with upsells, that is B.S. and I hate it as much as everyone else. But being able to reach out to them when there is a new version of an app, or something useful to say about whatever they purchased is valuable.
The hardest thing this day is not how to build a great app or how to write a great book. Our main challenge is getting noticed amid all the algorithmic shenanigans. Being blind about who purchase your stuff just adds more friction to that challenge.