I think that people need to get used to this new metagame instead of trying to find pokemon to fill garchomps shoes. People will always look for the strongest pokemon, and if it isn't there, they will look for the next best thing. As of now the next best thing is salamence, as you can see from the statistics. I think that people will eventually find ways to fit garchomp the one time counters back into the metagame, but in a different way.
That being said, I don't think it is right to judge garchomp's impact after one month. The metagame needs more time to adjust fro overcentralized to every pokemon finding a new role.
I think porygon-2 usage will go down really fast, and I am pretty sure it already has. If there were weekly statistics I am almost positive it would show a lot of p2 usage at the beginning of the suspect ladder and a little less at towards the end of the month.