Edit: I have just cast the fifth vote.

The edited question overlaps with a bunch of earlier questions:

• http://mathoverflow.net/questions/44095/arguments-against-large-cardinals
• http://mathoverflow.net/questions/48851/nonessential-use-of-large-cardinals
• http://mathoverflow.net/questions/35746/inaccessible-cardinals-and-andrew-wiless-proof
• http://mathoverflow.net/questions/29302/reasons-to-believe-vopenkas-principle-huge-cardinals-are-consistent

We can gather a very decent answer to the question by assembling the answers to the above. However, I don't see any exact duplicates in this list.

User Tip. Google is often a much better search engine than the MO search box. The list above was quickly gathered from the search site:mathoverflow.net large cardinals.

at the risk of fanning flames, what is The Big One?

Experience tells us that question 2 is inappropriate for MO. There has already been a summary thread on this kind of questions.

Question 3 is debatable. It may be entertaining to speculate about the effect of such a proof, but this is likely to resemble talk of The Big One. The impact on set theory would be disastrous in an unprecedented way, but the impact on mathematics as a whole would be comparatively very small. It is unlikely that this question has any kind of definite answer.