The first thing you need to remember is that Hawkeye is less than perfect. it gives an indication only, and can be as much as four inches out, and that's a hell of a lot when it comes to lbw decisions. The second thing is that unlike in football the rules explicitly state that the batsman shall be given the benefit of the doubt. Therefore, even if Hawkeye shows that the ball would have clipped leg stump, a not out decision is perfectly correct because there is doubt.
Umpiring is extremely difficult, but I've always felt that the umpires making an occasional mistake is as much a part of the game as a batsman chasing a wide half volley or a fielder dropping a catch. It's part of the game, and providing they aren't completely incompetent then I have no problems with errors whatsoever. Going to constant replays is a dreadful idea, everyone would appeal every time it hit the pad, and even then you hear the commentators say time and again that they're not sure. In that case it still isn't out, so I don't see where it helps us at all.
Even worse, we've seen time and again that a two dimensional image simply cannot cope with low catches. They ALL
look not out, even when a catch has been taken cleanly.
She's as nervous as a very small nun at a penguin shoot.