> not being able to save a file you've worked on because you ran out of credits would be a serious issue.

Yeah I agree with that, I think I went down the wrong path with credits = individual functions. Ive since seen that Autodesk does a credits system where tokens are used for time using the software. I think thats a much better idea than mine.

https://www.autodesk.com/buying/flex?term=500&tab=flex