People do have emotions and act on them from time to time. I don't think it requires any assumptions about Cutts's ambitions, but rather about how much effort it would take an annoyed spammer/black-hat to point some of their tools at the highly visible source of their frustrations.

Driving down the positive and negative value of links makes all SEO link building efforts consequently worthless. Which in turns increases the importance of other ranking factors.

You have a highly optimistic take on this. I think history generally shows that staying one step ahead of criminals/spammers/up-to-no-gooders is a never-ending struggle that does not tend towards positive resolution.

What "other ranking factors?" If you knew what those were, or even knew for sure they existed, the SEOs would too and they would be exploiting them. It's an entire industry whose sole purpose is to understand and game Google's rankings, and just as all this fuss about negative SEO shows, they're not losing.