It's one of the core things that has enabled the web to be as useful as it is. One of the things that draws people in, and keeps them using it.
Yes, there are problems with what we have. But if you break compatibility, you'll either not be adopted, or part of the crowd that the audience yells at for taking away their favourite things. You'll kill efforts and bury knowledge bases.
Taken to an extreme, "don't break backwards compatibility" has an insidious failure mode, which is "don't innovate." The car could not have come about without a willingness to break backwards compatibility with horse drawn carriages, plows, mills, hitching posts, etc.
The adoption of a radically new technology like this is voluntary, collaborative, and progressive. Provided it offers enough value to exceed the switching costs, there's no need to kill efforts or bury knowledge bases.