Realistically, a monthly subscription doesn't make any sense for this kind of service. I'm completely new to game-making and chose C3 after researching a lot of different engines. Sometimes, I'm really busy with work and don't have much time to get on and work on the game, and sometimes I run into an issue in the game and I have to take some time away to try to make the logic work. Then, sometimes I have a random breakthrough or burst of creativity and I spend days at a time working on it.
A monthly subscription would be more expensive in the long run, and would be more of a hassle if I had an idea and had to wait until I signed back up for a new month. It's much more cost-effective and beneficial to pay the incredibly low price of $100 a year and not have to think about it again.
Besides, I don't know about you, but I hate having a ton of random monthly subscriptions come out of my bank account each month. I buy netflix/amazon/etc gift cards at the beginning of the year just to avoid the monthly draft from my account.