Let's not get carried away now. That would make him top5 all time. He is obviously nowhere close to that. Let's wait a couple of years. He still lacks big wins, 2 Vueltas and one LBL is not enough to be considered a once in X years type of rider. And big wins is what sets the big champions apart from the rest. Also there are at least 5 riders in todays peloton who are ahead of him in terms of palmares. On the other hand it's true, that he has achieved all that in just 5 years as opposed to 10 or 15 years like the others did.Roglič, by now, is not only being considered a generational type of rider, he already transcended that. By being right there, competitive with the next generation of riders to come. Currently it is safe to say Roglič is a once in 25 years type of rider.
Then you have this crazy strong generation of young champions like Pogačar, Evenepoel, Bernal, and a little older ones like MvdP, WvA and Alaphilippe who are all comparable or bigger talents than Rogla and are all expected to win big and plenty in the future.
So you can't call all of them once in 25 years type of riders when they are all competing against each other at the same time. Now if you were talking about his unique skillset (combination of elite climbing, TT, punch, from March to October), now that's another story. Still, I am not ready to call him once in X years type of rider. Best rider of the last two seasons would be enough.