That will never happen, if they ever did that it would be an admission that there is something inherently wrong with their algorithm. They won't do it.
Nah. They let people 'report spam' in gmail -- they're happy to admit that algorithm doesn't always guess right. Also there's already a "give us feedback" mechanism for reporting bad search results, it's just too slow and manual.
People are used to spam buttons, and would realize that the lack of "report spam" would mean that Google might be marking too many things as spam if they think the user doesn't need a "Mark as spam" button.
People are also a lot less tolerable of spam in their inbox than they are of irrelevant search results.
On the contrary. As suggested by another commenter, it could be used to improve their algorithm. Greater number of exclusions would reduce their relevance.