How does non-notability decrease SNR? Does the existence of articles for non-notable people decrease the likelihood that people will find articles for notable ones? Does it lower the quality of other articles? As long as the articles for "non-notable" people are well-sourced and accurate, I see no reason why their existence lowers the SNR of Wikipedia. And if they are of low quality then there are more justifiable reasons for removing them than notability.
Yes, non-notability decreases SNR because it ends up crowding articles into disambiguation pages which makes it more difficult to traverse related articles on the site. It is generally preferable not to stumble across non-notable articles when browsing Wikipedia.
It is also more work for editors to 'maintain' non-notable articles over time. Having notability standards significantly reduces this burdain.
