I’ve actually been considering tackling all Alolan variations at some point, as each one of them is so vastly different from their outside counterparts that it’s often inadequate to simply address one form but leave the other completely out of the picture. (On a similar note, I’m still intending on covering mega evolutions for the same reason.) I just don’t entirely know when—perhaps when I’ve completed the main entries for each currently indexed pokémon.
As for whether or not non-Alolan grimer and muk are going extinct, that’s true … within Alola, anyway. It’s a mixed blessing, really. While on the one hand, a sharp decrease in any pokémon population is always unfortunate, but on the other, high populations of grimer and muk indicate severe issues in the habitability of a biome. A decrease in their populations, then, would mean an increase in the populations of many other species of pokémon.
Still, for those of you who may be fond of the grimer family, rest assured that there are still plenty of cities in the world with sufficient enough pollution levels to maintain healthy grimer and muk populations. Castelia City in recent years, for example.
The Pokédex According to Bill 