I think gold roger ultimately did have an ability. Haki is an important tool and makes a huge difference but ultimately in a normal human it can merely boost human abilities. Fruits powers on the other hand make the user kinda super human, it alters them in a way such that they can do things humans could never regularly hope to accomplish. Lets say we have a human who completely masters haki. Then, lets say that same human is given a DF fruit power and has moderate control over said power. Lets further say we have the non DF version against the DF version fight. Who'd win? It is extremely unlikely a DF would actually not benefit a fighter (worst case scenario the ability gain does not benefit combat but does not hurt it either) so in that sense what we have is that the DF version of the guy would win simply because he has a power. Haki in OP is not quite the same as in other manga. It won't give the user special or unique abilities for the most part. Heck, according to one of the OP databooks, fruits make such a ridiculous difference that one of the reasons BB waited for so long for that fruit is that he knew ability users would be in his way. In that sense the importance of abilities is so absurd that BB, being strong enough to fight shanks, still chose to give up his ambition if he did not run into the ability he wanted.
HAKI IS NOT A DF REPLACEMENT BECAUSE DF USERS CAN USE HAKI TO THE EXACT SAME EXTENT NON DF USERS CAN. Surely it is possible to stand up to DF users without having abilities but that is a rare thing to the point where so far we know of a handful of people at most who can rise to such a level. That said, those handful of people would still become more than exponentially stronger if they did take a DF. Imagine giving mihawk mr 1's fruit or giving garp a zoan fruit. How absurd would that be? It'd be ridiculous at least.
Lets imagine another scenario. How can someone plausibly fight WB in the sea? WB has control over quakes, which to a great degree means control over the ocean. If you are in a sea battle, what could plausibly be a greater advantage over that? As a matter of fact, WB's ability makes it so that any turf works as his home turf just for the inherent control he has over land and water. Can anyone imagine what would have happened in marineford if aokiji had not been a stupidly powerful logia users? They would have been hit by the full force of two huge tsunamis a couple seconds into the fight.
Magellan's physical capacity was not in itself a big deal at all, if anything it was severely lacking considering a worthless rookie like luffy actually managed to keep up with him. Magellan was the threat he was solely because of his ability, his ability made a stupidly huge difference.
Roger had position, power, wealth.... I have no doubt his sheer physical capacity was extraordinary and his haki was awesome but considering everything I do think he must have had a DF power to make it to where he did. It is certainly possible to make it as an ordinary human to a position of power (we have garp, shanks, mihawk, sengoku (assuming his power is like zoro's rather than a DF ability)) but having an ability just makes that much of a difference.
Another scenario which I thought would be awesome is garp with luffy's fruit. Just the thought of garp going into gear 2 or going into gear 3 with haki would be so ridiculous its not even funny. Heck, if he had had something like that perhaps "cornering" the pirate king would not have been as far as he could have gone.
