You have to be Spotify or Duolingo for me to comit to paying you everymonth. I pay a decent amount of money once, and I expect from you that you'll support the software long enough before releasing a V2 and I'll benchmark again if you're the app I want to pay for. Free market baby.
This is exactly why I'm going with a monthly and yearly pricing plan, not once off.
I use Thought Train daily myself and I can't imagine it going away.