Perhaps, but let's say this poll gets 500 votes "for" blocking TechCrunch and 300 against. What would that tell us? That there's a legion of TechCrunch fans/upvoters whose opinion matters less than a legion of TechCrunch detractors?
If so, is dismissing the opinion of people who actively vote up stories on HN a way to prevent the "decline"? I don't have an answer to these questions but if it comes down to blocking Web sites that are relevant to HN's gamut of topics, the answers could be important.
We shouldn't avoid paring stuff out to avoid the possibility of missing something. There's way too much news out there to take that approach, and the really big stuff generally gets posted multiple places.
Part of the problem is that HN's "gamut of topics", as you put it, has gotten too wide. There are lots of general tech news sites out there, this shouldn't be another. This is a narrowly focused community.
If so, is dismissing the opinion of people who actively vote up stories on HN a way to prevent the "decline"? I don't have an answer to these questions but if it comes down to blocking Web sites that are relevant to HN's gamut of topics, the answers could be important.