Why do you think selective filtering would ruin the front page? It's the equivalent of "don't click on Techcrunch articles", but it makes it much easier and removes the visual nag.
(For that matter, has there ever been a social news site whose primary method of filtering links was letting each individual user ignore things? It might be an interesting way to maintain a community.)
For the same reason cities suffer when people flee to the suburbs. If individuals can personally opt out of public problems, they have no incentive to work to solve them. In this case by flagging offtopic posts and criticizing mistaken ones in the comments.
You're equating a physical reality with a virtual one. The whole of the Internet is about filtration. There's no "real" location that grows worse or that requires effort.
Besides, a per-site filter won't stop offtopic posts from being flagged, or debates from starting. They'll only allow certain people to ignore certain posts, which those people want to do anyway. I for one wouldn't block TechCrunch, and I'm certain many more would keep reading it as well.
