Did I ever say that? No. I'm just saying that makes Grass an even less useful type, and it's uses right now are pretty minimal. Breloom and Amoonguss are really the most common due to being the only usable Spore users(Sorry Parasect that 8x Fire weakness screws you big time.) Not only would it lose superiority to 6 or so Pokemon, it loses neutral coverage against 11 more fully evolved Pokemon(Not including Ice Arceus) which believe it or not, can change up the Metagame massively and make certain Grass types weaker. Nerfing a type that doesn't need it to buff another type is not how you do it at all.
I never said pure Grass was good, I was using it as an example as adding on another type can weaken or strengthen a Pokemon, I was using the typing itself without any benefits or burdens. The only reason those Grass types are in OU is mostly because of their defensive capabilities. M-Venu, Amoonguss, Breloom, Tangrowth, and Ferrothorn are all defensive, only one of them are pure Grass, Serperior is the only offensive one, and to be quite honest, is easily countered by Sap Sipper Azumarill, Klefki, Jirachi, etc... especially the ladder two which can tank a hit from Leaf Storm and in return can paralyze, or immediately paralyze with Prankster Klefki.
Grass is doing okay. Making something else resist Grass and then Grass will be doing bad. Right now Grass, Dragon, Fire, Poison, Flying, Bug, and Steel ALL resist Grass. Add Ice and that's a grand total of 8 out of the 18 types that would resist Grass, nearly HALF of the available types. While Grass hits Rock, Ground, and Water super effectively. It also only resists Ground, Grass, Electric, and Water, and gets hit super effectively by Ice, Fire, Bug, Poison, and Flying.
Take that into consideration, okay?