Certain games are just right where they should be. It's like a sport, eventually people just find the right balance with the rules and everything just gels. DOTA is one of those games that just plays great even with all the limitations of the WC3 engine it's still chugging along on. DOTA 2 will have improvements and changes but they're keeping all the things that make DOTA great while trying to fix the original's flaws. Just because you can change something doesn't always mean you should.
I have a hard time believing DOTA is the best the series can get since it was the first MOBA game (at least the first one that gained large popularity). I am sure there is a lot of room for improvement.
I agree with you that, while game should push forward and innovate, they should also keep what made the game great in the first place. Whether LoL is doing that well or not, at least they are making an attempt. What I don't like is when developers are blatantly releasing the same game, especially when the developer admits it.
That being said, I am really not a LoL fanboy. I have grown tired of their inconsistent balancing issues. They seem to want to balance champions for the lower elo crowd and that doesn't work well for competitive play.