you describe in the FAQ that the pro version is not a subscription. That may be a good thing for many users looking first at their wallet but I would like to propose to reverse this.
Over the years, I saw many small scale projects stop development because the lack of funds. Developers tend to some day look for a "real job" covering their expenses and not to struggle alone anymore on their own app.
A one-time fee purchase forces you, I guess, to focus on the acquisition of new customers. No new customer, no money. As your user base grow, so do you costs and support time, but not your revenues. It is a vicious circle. Correct me if I am wrong, @timo.
A sustainable way to earn money is through a subscription. Goods that are purchased once and then do not cost any money do not exist in the physical world also. If one buys a car, he must include the cost of the fuel and of the repairs over the years.
My goal is that diarium stay available to its users in a durable way. A growing user base means more troubleshooting and more costs. All work deserves a salary. I would love to see diarium get always more polished and feature rich. Development stop and/or support stop would definitely be bad news.
Nearly all of the android apps I pay for are subscription based: my keyboard, my weather app, my cloud storage app, my todo list, my music. Every year, I am glad to pay a fee for them. That way, I am (mostly) assured that the features I love keep being supported.