I think, that developers ask for periodic payments to keep their interest going and sustaining themselves financially.
While I really don't like, and try to avoid, "subscription-based" software, this is a valid point. Especially in the case of an email app, which is used many, many times a day. If you use it, you should pay for it. I paid once for AM, long before Mobi took over, but use it more times daily than any other app. And since I paid a one-time fee, it's the newer paid users who are subsidizing the likes of me.
That being said, I will not use cloud-based apps for all the reasons you shouldn't use them. I was very sad to see that SM was cloud-based because I really like what Kostya did with AM. And the support he provided was really great. While Mobi has kept it going, their support could only be worse if they had no presence on the forum. It's once every week or two that we see someone stepping in to answer a question.
So I will reluctantly stick with AM while I test FairMail.