Have a look around in forums, reddit and other social networks:
There is no recommendation for AquaMail anymore. Price for pro key meanwhile has increased to 40 €.
So try to predict the future:
- Free Version is not usable, too many ads
- Many users own the pro key already
- Only very few people will purchase the pro key or do the subscription.
MobiSystems won't make money. This is why there is difficult to develop and to provide best support.
A lot of apps died already this way.
If MobiSystems would tell existing pro users about the situation and would ask for a new one-time donation of 5 or 10 euros to support further development, then they would make more money. The pro key could be 10 €, so they would get new customers.