The proper way to express this formula would be:
T(n) = 50 - F(n-1) + F(n)
-F(n-1)+F(n) would average to zero over every two blocks (for obvious reasons), so all you're really proposing to do is just reward 50 BTC per block, forever.
Okay then. With difficulty increases, calculate the coinbase reward for the upcoming 2016 block period as the average sum of TX fee in the previous blocks.
ALSO!: My formula calculates the number of NEW coins printed. the REWARD averages to 50, yes. the number of NEW coins does not.