Let me start by saying that I wish you the best of luck and success with the app.
With this said, I have many subscriptions that "costs less than a coffee per month", to use a popular excuse. The problem is that I pay $3 here, $5 there, $7 for this, $10 for that... and when I add it all up, it's a lot of coffee, if you know what I mean. And at this point I don't see why I should pay for a program that will work without your intervention while I'm running this macOS version (or even across multiple macOS versions). I'm not using any of your resources/time or costing you any money.
I completely understand that you want to be paid for your time if something breaks and you have to fix it, but you don't need a subscription for that. Some apps give you x years or x macOS updates for X dollars. After that, if you want a new update, you pay for another x years/pay for the latest major update (or something like that).
If this used any server side stuff or required regular updates, then I would understand the subscription. As it is, it's a bit like Adobe software... I don't need it enough to pay for the subscription.