I don't quite follow your logic, and I don't think you follow mine. The fact is, as it currently stands, even if we took your theoretical and say Applin had Grassy Surge instead of Grookey, suddenly we would have Applin as a Terrain-setting lead, before either eject-packing out or just getting one-shot due to its frailty.While I don't play LC, this isn't the best way to look at this. Lets look at a certain emotion ability, Moody! It is broken on every one Pokémon and that's why its banned, if we were to give a Pokémon like Applin, it won't just rise to the top of the metagame due to the fact that it doesn't have the stats to use it itself. LC won't ban an ability if its only breaks one Pokémon like the example of Gorilla Tactics, it wouldn't break a lot of Pokémon, so it was banned instead of its ability.
The point that I am trying to make is that whatever happens, we don't have any terrain-setting ability in the tier, and the only options to counter it is Defog, setting a terrain of your own, or letting it last out. If your opposing Grookey has Wood Hammer and Grassy Glide, your counters are limited. Only two Pokémon in the tier have Sap Sipper: Azurill and Goomy, and the two abilities that can stop priority moves, Dazzling and Queenly Majesty, are not available options either.
More than that, Grookey has U-Turn, allowing it to come out, set up the Terrain upon entry, and then switch out while dealing damage. Do I think this is broken Out of context, I would say no. Rillaboom does make it into OU thanks to this set, but again it is all thanks to its Hidden Ability.
I think the biggest issue that needs to be addressed is the team synergy. Thanks to the options listed above, it can set up its teammates, who may also have Grassy Glide, to come in and deal with the enemy as well. Grassy Terrain is a major boon for any team that has it, and the fact that Grookey has a solid moveset to work with is a solid plus.
People are arguing here that Grookey is over-powered, and talking about a Grookey ban. I am saying that Grassy Surge is the specific tool that makes it overpowered, and that THIS should be the specific target of the ban until such another terrain-setting ability makes its way into the tier.
Generation 4 Little Cup was the first time weather came into play, thanks to Snover and Hippopotas. If it wasn't for Psyduck being an option with Cloud Nine, these two large threats would have had full control of the tier. The point I am trying to make with this anecdote (which I am just grabbing from Smogon so my understanding of the situation may be a bit off) is that for certain team formats, such as any weather, there were effective ways of countering them, mainly Cloud Nine and OTHER weathers. Neither of these is something we have here for Grassy Surge.
...sorry, rant over. Hope this clarifies my stance.