But that still has the problem that it makes his 2012-13 level look ridiculous.
At this point, if he's doping, and like many I strongly suspect he is, Froome's in a Catch-22. If he cleans up and his results drop away, it makes quite clear that his 2012-13 results were doping because he isn't able to continue to perform at that level. But if he continues to dominate with performances that are regarded as suspicious, as his 2012-13 performances were, then he continues to be regarded as a suspicious rider and probable doper.
The only way I could see for him to step back a level without causing suspicion would be if he had an injury. An illness won't do at this point, because an illness has already been used to justify his increase in level, and people are inherently suspicious of illness stories surrounding Froome now. It would need to be something like crashing out of a race and missing time to injury, so that people could see it wasn't smoke and mirrors, then returning not as strong a rider. And I'm sure all but the most rabid of Froome-haters would prefer him not to get legitimately badly injured, which takes that "solution" out of the equation.