I bought my Pro license in 2013 and when I purchased it the agreement was that for life I would get all future updates improvements and features for the program.
That's not true, for any app.
When you purchase an app as one-time-purchase, you buy the existing features. Of course you hope that the app will be maintained. But there is never a promise about that.
I don't like it, but it's ok to develop new features only for new payments. (I would have paid 30 € as a one-time-purchase, but not subscription).
And it's not ok to cut features. As it seems to be actually, this will not happen for pro users. THAT is a promise by AquaMail team. We will see what will happen and how it will happen.