To me, MvdP won his three big races against all the top one day racers, who targeted those events, whereas Roglic's Giro was certainly impressive but not against the best GT riders. Not sure how the Vuelta will play out but that's how I see it right now.
But he did lose the one that is supposed to suit him most (RvV). ;-)
And in Roubaix there was also some luck involved (there is a good chance he would have won nonetheless, but we can't know for sure).
In Milano-San Remo and the Worlds he was clearly the strongest of the top dogs though. So I'd say at the moment it's between him and Vingegaard.
Now he hasn't, but if he wins Vuelta and Lombardia what would you say then?
Imho Roglic can never win it. Sure he won the Giro and some smaller stage races, but only because he avoided the TdF. And love it or hate it, the TdF is just way more important.
Even if he wins Giro and Vuelta, well, he didn't go to the TdF because he would not be able to win it anyway. Therefor his season can't be better than the one that actually did win it.
He also didn't win any of the other races that matter to make up for it - like Remco did last year.
Only the absolute top races should matter: monuments, GT's and Worlds/Olympics. He can win as many Tireno's as he wants, but in the end that's not what it's about. One week stage races are to the GT's what the E3 classic is to the Tour of Flanders. Nice, but not what actually matters.