Letting them all use drugs won't make any difference, you'll have to give everyone exactly the same gear, same dosages etc. but that won't happen since people will still try and cheat by using more, taking different dosages, using different drugs that the rest aren't using.
So in the end we'll be back where we started, they'll still need to be tested to try and keep everyone honest.
It's a part of life that we just have to live and deal with, testing has currently made it as fair as we can make it.