In the Arcanine calculation, you have to either take intimidate or burn into account as an opponent switching in Mega Kanga while Arcanine is already out is susceptible to burn while Arcanine switching in automatically causes a one stage drop in attack. The OHKO is only with Stealth Rock which is easier and easier to get off the field with buffs to Defog and strong rapid spin users like Starmie and Excadrill. Also, Arcanine has access to Morning Sun which received an indirect buff this generation with the nerf to weather. Forcing Kangaskhan to use Power-Up Punch to bump it's attack stat back up to an acceptable level allows you too much free time to put damage on him that will stack up quickly with burn or simply stall him out. The classic Parashuffler Dragonite set also handles Mega Kangaskhan really well paralyzing it on the first turn until you get a paralysis proc and can safely use dragon tail to phaze him out. Once Kangaskhan is partially weakened and crippled by paralysis, he isn't nearly as scary as he was before and has to rely on Sucker Punch to be able to move first. I know I'm mentioning a lot of defensive mons, and I'm not suggesting that a team run any specific combination of them. I have found stall and balanced teams to be my favorite, and these are just a number of examples from the various teams I've tried.
Look at the calc again. It does take into account intimidate, and assumes Arcanine is switching in on power up punch. If Arcanine does so, the extra damage from PUP will ensure a one hit ko, regardless of rocks or not. Heck, Kanga has a shot at One hit koing without any residual damage (again, look at the calcks again, which you misread). At +1! Arcanine has no buisness staying in or switching on a Kangaskhan with any boosts, and if already weakened, you might as well fodder it to the wrath of Kanga. And what's the point of someone switching in Kanga on Arcanine, I would much rather switch in my Rotom, underspeed with Volt-Switch, then go into Kanga for example.
As for my opinion on mega Kanga, it is simply a combination of too much power and bulk, combined with a neat 100 base speed. If Kanga was slower, I'd assure people it would be so much easier to deal with, we could switch in our Physical walls, that, with minor investment in speed, could cripple with status or weaken Kanga. Usually, powerful, bulky pokes get a bad base speed or a crippling weakness, to compensate for their extreme offenses and bulk, allowing you too check them. If the poke is powerful, but has a lot of speed, they are usually squishy, to make it easier to wear them down. Tyranitar is a perfect example of the former. And Kanga is basically a Tyranitar, with an arguably better typing, that is not starting out on 60 base speed, but 100. This immediately causes problems, because now, no wall can hope too outspeed it, and fast pokes need to have a resistance too Sucker Punch to stand a chance. And when the thing has the ability too choose what walls it can't take on, that makes it practically impossible to be Mega Kangaproof. The only way too check him offensively is faster priority or Sucker Punch resistance, only walls who can take him on are ghosts and Skarm, and all of them can be screwed over by either teammates or Crunch.
Now, does this make him broken? In the current metagame, I think he is, the combination of bulk, power and speed is unrivaled in the OU tier, and it's not very difficult for Kanga too run through the meta. You need dedicated counters to take him on, and even they are shaky. He might stabilize after some time, when Pokebank arrives with stuff like Terrakion, but there is no question that Mega Kanga is the best mega, and there is little reason too not use him. So the most healthy option is probably to ban him.