I have seen all the excuses of not implementing pgp and s/mime in these forums.
I would phrase the business case like this:
I recently started up a new company in the financial sector. In the sector there is a big need for encryption of private information due to security and fraudulent behaviour of some dark elements.
Aquamail is an excellent app and appeals to most people due to a good and worked through gui and engine
Maildroid, K9, Squeaky mail, Outlook, outlook.com and many other options out there is nothing in comparison to Aquamail. I have tried them all when I searched for an app that can do pgp or s/mime. Most of them seem to be coded by coders who does not understand the need of eg syncing the sent box when on imap, crappy gui, no proper scheduling of emails and a lot more.
Aquamail has a unique situation where you can grab a big market in both the financial services if you implement encryption no matter of what encryption you select. Also the health care sector, government etc are in need of the encryption and most select to not do it on the phones as there are no good alternatives.
You can grab a bigger market share if you implement it in your app. You can even replace the crappy outlook app that I used to like.
Private persons are good, but think businesses instead. This where you market is.
And you can increase the license price due to this also.
But before you upgrade, please use a proper pgp management app. There are many monkey pps out there that creates more issues and hassle for the users than if you use eg open keychain as the key manager. Agp is crap, it cannot encrypt files for instance and some others are like as they gave not left the Ms Dos stage.
You have made an excellent job with the app, take it to the next level.
We had to pick one of the crappy apps above due to that yours didn't do pgp and s/mime.
That will most likely lead to that we will start using those apps instead and skip Aquamail no matter of how good the gui is. Our clients means business to us and therefore we need to prioritise security for our customers more than a perfect beautiful gui.
This is a bigger risk for you than the few that select you, ie people leaving your app if you do not listen to the need if proper functionality.
So, I suggest you get your act together and move Aquamail to the next level and stop the excuses.
You can make Aquamail a profitable business if you just listen to the users.
I do foresee a risk that your app will disappear as you will not grow it further as people rather go for outlooks app and that app will catch up with yours. That's what has happened over the past decades with other apps if they don't keep beating Microsoft.
The choice is yours only.