Not all users of Knock off screw Blissey AND Chansey.
I can name a few
Tangrowth
Mandibuzz
Mixed Zoroark
Tauntless Sableye
Tentacreul
I have seen these few pokemons a lot in the game, but your point stands, MOST knock off users screw Blissey and Chansey.
Also people are not acknowledging the importance of Leftovers. Leftovers means you are taking hazards better, it also means that the turn you use Protect to Toxi-stall or heal through Wish, means you are gaining 6.25% percent of your health back, and that's a lot in blissey's case. Leftovers means you have better chance of surviving, Leftovers means that when the opponent is switching you gain 6.25% of your health back.
However, Chansey takes hit MUCH better, and can do some stuff better than Blissey, like Counter for example, and Soft-Boiled means you are gaining HP that carries a lot of Def/SpD, Chansey is slower though, and while that doesn't matter in most cases, it sometimes does a lot. I think passing 5 HP less isn't extremely irrelevant and most pokemons tha
I think Chansey fits good in B, but is it me or is there no ranking for Blissey? I definitely do think Blissey should get B too, as they have almost the same niche but using different means, IMO, both are good.