The problem with those offensive checks is that none of them can actually switchin or reliably check Megagross (you also kinda glossed over the fact that there's hardly any consistent defensive counterplay either):
- doesn't take ZHB well at all and a flinch or MM attack raise makes it a non-check.
- Scarf is terrible. It gets outsped by every other scarfer and can't break any bulky grasses or waters like Slowbro and Tangrowth. Scarf Rend doesn't even OHKO, regardless of Jolly or Adamant nature due to MMeta's immense bulk.
- Scarf and Rocky Helmet sets are garbage; they lose to common stuff like G-Darm and stuff they used to check like Lopunny forces it out now due to new mechanics. Not to mention they still take a lot of damage after Intimidate and can't even OHKO MMeta back if you're running the defensive set.
- Gets worn down easily and takes a shitton from EQ. Not to mention this already struggles in a meta where Ash-Gren is everywhere AND is Mega Metagross' favorite partner.
- Sucker Punch can't OHKO without an SD, and Megagross just chunks it or outright murders it with EQ and Hammer Arm respectively.
- Neither can switchin and Tapu Fini is everywhere to beat back Ash-Gren and Defog its Spikes away.
- not relevant enough in the meta and it has no recovery.
- has to waste its Z-move to attempt to OHKO just to be walled by common teammates like Phys. Def Tangrowth.
- Every attack that isn't Mash just damages this way too much to be a reliable answer. Fini is everywhere too, which means the bulky variant will struggle even more, especially since it can't fish for burns with Flame Body while Misty Terrain is active.
We already have enough ways of beating stall too. Rain has started ditching Vish for Manaphy, and Tapu Fini prevents stall from progressing much at all. It's worth noting too that a lot of the tier's best stallbreakers, such as Tapu Lele and Mega Mawile are pretty difficult to justify right now because they lose to or are outclassed by Megagross among other things. Metagross itself is NOT a stallbreaker, but its vast coverage, speed, and bulk among other things makes it hard to justify alternative options as mentioned before.
Even if we didn't have enough stallbreakers, it's not really a valid argument as we're not focusing on how the meta will be in the future without MMeta, but how it is in the current meta.