You think I could not identify all the functions in some software if I poked at it with IDA pro for 15 years? Probably depends on what exactly we are talking about, is it calc.exe or an entire office suite? But I will give you that I would probably never complete the task, I would not be willing to spend 15 years doing this. Realistically I would probably be interested in some specific functionality and identify the functions involved and ignore the vast majority.