DotA is already a far cry worse than LoL in terms of community.
Actually, a ton of f2p LoL players would make the community BETTER, in my opinion. From my experience playing in high tier games with the same DotA1 people I've known for ten years compared to when I queue with new players and get games full of LoL players, I have to say the DotA1 vets are much nastier and less fun to play with.