??? DPP Gyarados didn't need priority because it consistently had a dance or two and I don't know why you even think this matters. My post was articulating that DPP Gyarados was a very similar beast to MegaKangaskhan and it was never banned. The way you beat both of them is by making an all-round strong team that is good against the majority of OU and predicting well against them during the match. If we didn't ban DPP Gyarados why should we ban Megakangaskhan?
In before more replies about it having no counters and similar meaningless drivel
Mega kanga has a neutrally stronger stab, can run 4 attacks, can bypass focus sashes, and is similarly bulky to gyarados (especially without intimidate). Before you go screaming about intimidate mattering when talking about bulk, most things that gyarados would come in on to set up on would switch out anyway, so intimidate is often irrelevant. Gyarados is also weak to stealth rock.
Additionally, mega kangaskhan gets a +2 attack boost in one turn as opposed to +1 (don't even think of arguing about dragon dance's speed boost. you can't be that dense, can you?), after which return, priority sucker punch and its coverage move of choice will allow it to OHKO (or 2HKO with impunity) almost any typical OU poke. +1, and sometimes even +2 gyarados did not seal the game as easily as you make it out to have done so. I don't recall much from my DPP days but (and I'm not going to claim with absolute certainty on whether or not it should have been banned) vaporeon, swampert, donphan (with sturdy up) and skarmory were extremely common back then which all put a stop to gyarados sweeps.
What reliably stops kangaskhan? We know that pokemon are banned if they, under very common conditions, are reliably able to set up and kill a large majority of the opposing team without being able to be stopped (think blaziken). What reliably stops mega kangaskhan? choice scarf rotom's will-o-wisp? Sableye? Cofagrigus? Actually, assuming m-kang PuP'd the rotom on the switch, he can set up
another PuP on the switch out of rotom and still wreak havoc on your team. Do I really need to go into Cofagrigus? Sableye is the only real counter to this thing that can be found on a good amount of teams. You're practically forced to run one of these three if you hope to deal with m-kang with minimal losses to your team. Talonflame (after some previous damage) is one of the very few checks to this thing.
You mentioned that it wasn't bad that Mega Kangaskhan incentivizes one to use infrequently used items, namely rocky helmet. This isn't what M-Kang is doing. M-Kang is forcing you to do this if you don't want to lose to it. Sure, rocky helmet isn't the worst item to use against any team, but it should not have to be considered to begin with merely because mega-kangaskhan exists. It's also only really worth justifying on garchomp and ferrothorn, and kanga often carries earthquake..
Is it impossible to win against mega kangaskhan? Not at all. In fact, I think I've won more games against it than I've lost to it in pokebank OU. Is it significantly harder to win against it than it is vs many other megas/ physical sweepers. In my experience and apparently that of others-- hell yes.
I mean is it really okay for a pokemon with a non-move-locking choice band that goes past sashes that can set up vs anything but ghosts (and sometimes even on them with the right prediction) to FURTHER increase its attack really okay? Jesus christ. Imagine if CB Azumarill could switch freely between its moves with a +2-4 boost and had the speed that kangaskhan does.