I wonder what would happen if a contender were diagnosed in this manner, particularly someone like Froome who is very likely to win. Would he drop out, and abandon his chances of a title, or take a chance that two more weeks at this early stage wouldn't make much difference--and getting as much treatment as possible under the circumstances? If you think someone would be a fool to gamble like this, remember that athletes frequently take much bigger gambles with their lives, indeed, that fuels a lot of the doping. 
In any case, there has to be a cutoff point, a stage in the race where a strong contender would continue to the finish. I can't imagine anyone leading after stage 20 dropping out just before the final stage to Paris, for example. But how far into a race would a contender have to be before he would make the decision to continue? I can see a point where it would be a real conflict.