Wow....Although this is my first ever post, I honestly believe that Mega-Kangaskha shouldn't be banned.
First of all, in my opinion, this seems like it'll lead to a slippery slope. Next thing we know, we would want to ban Azumarill, Mega-Mawaile, even Diggersby for their abilities to reach sky high attack values while also retaining priority moves.
Secondly, Kangaskhan has a very predictable move pool which comes in the form of the Power-Up Punch, Sucker punch, and Return with a 4th varient move (from what I've seen) which more or less only may knock out one of your pokemon at most before you switch into a check/counter. Which comes in the form of various Steel, Ghost, or Fighting Pokemon depending on what 4th move it uses (faster fighting types usually being able to revenge kill it, like the Muskuteer Pokemons). I've also seen many Rocky Helmet Ferrothorns counter Mega-Kangaskhan multiple times, taking recoil damage twice. Sure it's a Choice Band with the ability to change up moves, but the same applies for the above pokemon I just stated and Life Orbed Pokemon usually hold the same fire power as a Parental Bond Mega-Kangaskhan.
Rocky helmet being a counter actually supports banning it as it is leading to OVER CENTRALIZING. Also, can LO holders brake sub and sash? NO. And btw, most of the fast fighting type pokemon you mentioned can't switch in on Mega kang safely. And predictable? Just because Manaphy was predictable last gen doesn't mean it wasn't OP
No way will this lead to diggersby or any other of the pokemon you listed ban.