I think if you're talking about greninja, you're talking protean. Ash is much more predictable and is usually choice locked and therefore it's exploitable. Protean is a monster that KOs several defensive mons depending on what it's carrying. Notably, it KOs scizor with HP fire - a mon considered the premier metagross wall. I don't think it's unreasonable at all to argue that a meta without greninja is a meta that is also more easily able to handle metagross. And not to derail the thread (I won't post further on the subject), but greninja is likely to be banned for a lot of the same reasons pheromosa was banned.
Metagross isn't the same beast. You know 2 of it's moves right away, you know it's physical, you probably know a third move based on it's partners and you likely know the last with a tiny bit of scouting or observation. If you know metagross' set it can be stopped. Meanwhile, with protean gren I find myself switching into a potential answer in the hopes that gren isn't carrying that particular coverage move. Pheromosa was banned because it had so many options and little risk to using it that you would be forced to pray you get it right - or lose a mon or the game entirely. Metagross is just so much more predictable and I think that's a huge distinction you can make between it and the other S or former S ranked mons in the tier.
Metagross isn't the same beast. You know 2 of it's moves right away, you know it's physical, you probably know a third move based on it's partners and you likely know the last with a tiny bit of scouting or observation. If you know metagross' set it can be stopped. Meanwhile, with protean gren I find myself switching into a potential answer in the hopes that gren isn't carrying that particular coverage move. Pheromosa was banned because it had so many options and little risk to using it that you would be forced to pray you get it right - or lose a mon or the game entirely. Metagross is just so much more predictable and I think that's a huge distinction you can make between it and the other S or former S ranked mons in the tier.