A film/developer combination which needs 20 minutes of time does not exhaust 1/20 of it's power every minute. Rather than being a linear relationship, I would think it is a sliding scale, where 40% of the development occurs in the first 5 minutes, 20% in the next 5 minutes, etc., etc. which is why reducing or increasing developing times has a strong influence on highlights and contrast as these are the areas that need the most development time. So, while I understand what you are trying to do, I suspect you would have incomplete highlight/contrast development under your theory.

Just my opinion, I am sure there are more scientific answers.