The problem is that SFOS calendar tends to sometimes skip reminders 
Do you really believe that those €5 a month from the same number of people as who so far has been paying one-time upfront fees can really bring an amount able to “keep the company alive”? With this tiny user base of just literally thousands of paying people, I’m not sure if it’ll be enough to hire one new developer.
As you might have noticed, the problem definitely isn’t with the amount. It’s not about the money.