When I was coaching high school running I read A LOT of stuff trying to improve and one thing that was pretty apparent was that it takes about 8 years of consistent, high level training to reach your peak. High school phenoms who break 4 minutes in the mile/1600 usually started pretty young and are probably six or seven years into their training. That is why so many highly touted HS runners go to college, but don't dominate. Their peers that were running 30-40 miles a week catch up and sometimes pass them.
Is that where Pogi is this year? I have no idea. This year was like a perfect storm of goodness for Pogi in training and racing. No illnesses that I read about, no crashes, a relatively weak Giro field (that meant he did not have to go too deep) and then his main Tour rivals all crashing in April so that their prep was affected to different degrees.
Other riders have tried it and failed more recently. Contador won the Giro but then just was not there and could not beat Froome and Sky. When Froome tried it, he could not beat Geraint Thomas. It is really hard because most riders focus their year on either the Giro or the Tour. I'll be interested to see if he tries to win the Giro-Tour annually, because one has to think it will be much, much harder.
After he reaches 10 GTs (5/6 x TDF; 2 x Giro; 2 x Vuelta) wins he can focus on Paris-Roubaix.
With a favorable wind on the final climbs he can win Milano - San Remo earlier.
Tadej is the most incredible rider cycling has ever seen...sure he could win Roubaix
He will have to gain about 15 pounds to pull that off. Hope he saves that for a little later in his career.