I've seen a lot of people make good use of Mega Charizard, Mega Llucairo, and Mega Gengar (which I was personally against banning). It's arguably the best but I don't know if it's fair to say it completely outclasses them.
All of these (well, maybe except M-Gengar) have noticeable drawbacks which serve to balance their power. Charizard and Pinsir worry about Stealth Rock, Mawile is extremely slow, and so on. They also have very defined checks, that are viable in other OU contexts. Even Lucario has the drawback of being a glass cannon. Kangaskhan has
no drawbacks, as explained in great detail earlier in this thread.
Crunch means getting rid of EQ or Sucker Punch.. making it vulnerable to other Pokemon. What if the opponent has a ghost on the team but it isn't out? What if it is something bulky like Skarmory that can Whirlwind you out after Power Up punch? it isn't always that simple.
If you saw my list of calcs early in this thread, you can see that Skarmory gets 2HKO'd by +2 Return. An attack
that Skarmory is supposed to resist and tank. It's probably safe to say, it's going to die to an attack that it doesn't resist. (And why can't you have Sucker Punch and Crunch at once? Lots of Kangs do this.)
I mentioned this in the post, it isn't about getting 2-3 Pokemon there for the sole purpose of countering Mega Kangaskhan.. I used Rocky Helmet Skarmory even in gen 5 when it wasn't around. These are already good Pokemon that perform well even when Kangaskhan isn't around. Kangaskhan has an influence on my team builder just like lots of major OU threats do. I might choose one Pokemon over another because of Heatran or Dragonite or another powerful OU Pokemon.
Using those things isn't really the problem alone. If you find Rocky Helmet useful for other things, by all means use them. The problem is when they become widespread just to counter one thing
and used over other items that may be more optimal for many other teams, which
clearly is happening. Using Rocky Helmet on Skarmory alone has clear downsides over Leftovers (no passive recovery), or Shed Shell (Magnezone), yet people feel the need to put them on
even when they would be disadvantageous in any other circumstance in their teams, because Kangaskhan. At the moment, Rocky Helmet is being used not only on Ferrothorn, Garchomp and Skarmory, but on the most random shit imagineable (Furfrou? really?), a classic symptom of overcentralisation. Back in the Excadrill metagame in early BW, you could've used Air Ballon on absolutely everything and said, they serve other purposes. Fair enough, but you could've used that item slot on something else, which would've served a better purpose for that Pokemon.
Smogon, in general is against complex bans. In theory, yes, I suppose you could have something silly like "You can use Mewtwo on your team, but only if you also have 4 UU on it" or "You can use Ho-oh but only if you also use Slugma" and the team would probably be "balanced". Drizzle + Swift Swim was an exception.
I don't think you've understood, what I said. I was saying, that megas shouldn't be treated any more different than any other Pokémon (or for that matter, item. A broken megastone is very comparable to Soul Dew; it makes an otherwise manageable Pokémon unmanageable.)