Well, WC3 didnt have to compete with Brood War, CS didnt have to compete with Quake, as long as the games provide a seizable different focus (micro vs macro in the former example, 5v5 vs 1v1 in the latter), there's no need for competition. Despite a lot of people calling the games similar, LoL and DotA are very very different games. DotA is way more strategic, with a lot of games starting with a team having the advantage simply because of the draft, while LoL is way more demanding when it comes to technical/mechanical skill. Why not enjoy both?
(I personally don't enjoy LoL, but thats because game complexity is key for me and competitive LoL has little to none, but that's just me)