Running fast is harder than the TDF wrote:
The thing is, a good runner, or even a 31 minute runner can become a TDF racer and do well. It has happened a few times. No pro cyclist ever became a good runner. The doped to the gills LA was about a 2:50 marathoner, and even before he became a doping cheater was slower than a 5 minute miler in high school. The very best a cyclist has ever done is about 2:40. No matter what all you self-haters want to believe, it is easier to learn bike handling and pack riding than it is to run with the efficiency to break 15 or 31. This has been proven. A runner can and has gained the power and skills to ride fast, but a cyclist can't gain the aerobic output and stride efficiency to run fast.
This has not been proven, if anything, elite triathletes prove that is is probably possible, but no one does so. Take folks like Alastair Brownlee or Alex Yee. Both have run competitive track 10000s while being triathletes (low 28s) while also having the ability to ride fast on the bike. Sure, they are Olympic distance guys so not like they are ready for the Tour "right now", but I think it shows if they decided to go either way, pro runner or pro cyclist, they probably could.
Jan Frodeno ran a 2:40 marathon after averaging 27mph for 112 miles in an Ironman. He clearly has the ability to at least be a domestique, and maybe he could never run 2:10, had he chosen a life of running instead, he undoubtedly could have been down in that area.
To summarize, I'm not suggesting every cyclist is capable of being a great runner, or every runner able to be a pro cyclist, but there are folks that could have went either way.