Something about suckers and minutes.
Look, dude, if we could treat the underlying disease behind the symptoms (of which spam is only one of many, and is exactly why it is successful) then we wouldn't have war, poverty, racism, sexism, religious extremism, rape or murder. I'm serious. Spam is insidious (and equally brilliant) because it preys on people's insecurities and neuroses. Smart people get taken in by spam all the time.
Further, I take contention with your assertion that phishing is easier to defend against. If it was then there'd be no such thing as espionage. It's all a confidence and trust game. Sure, it might be on a computer, but the tactics are no different than they were a hundred years ago. How do you propose to prevent granny from responding to an e-mail from what looks to be her bank asking for her username and password?
As far as web-of-trust e-mail goes... welcome to 2002 (most likely earlier). Web hosts and software packages have long been offering this. You know this, right? And in regards to "accept mail from my Facebook buddies..." Man, seriously? So, you're saying that Facebook, which is EASILY hackable/phishable, is a better/more secure form of e-mail? What happens when your buddy's Facebook gets hacked and he sends you a well designed scam? You're more likely to trust it coming from someone you explicitly gave permission to e-mail you, your friend, so is it really anymore secure?
The best (only, really) defense against spam is a well trained Bayesian filter combined with a well maintained ISP/host-level blacklist. Also, knowing what not to do, what not to click on, etc. Even then, shit happens. That's why we have laws. Time to write better ones and then start enforcing them. Criminals get smarter, skirt the law. Rinse and repeat.