What are the reasons to not open source the Android App?
The encryption code is open source. The app itself remains closed source because otherwise there is no way they could earn money with it. You could just take the code and compile it for yourself.
Developing takes time and paying for the app is one way to reward that.
It’s good that they should be able to make money from it, but according to that same argument why not charge for the desktop version then? The vast majority of users of software like this that aims to make complex processes simple are not the sort of people who are going to go run their own compilers; most of them would still pay for the app. And for those that do run compilers there could also be a donate feature in addition to app sales.
So I understand your argument but I don’t think it’s valid to justify saying on the front page that it’s open source when really it’s only partially open.
The desktop version is open source. Completely.
If you want an open source mobile client, feel free to create one. There’s absolutely no need to use the paid cryptomator apps if you don’t trust them only because they are not open source.
I totally disagree with your argument that most satisfied users pay for open source apps. There is a reason why so many open source projects are ending because of financial reasons.