Deathtrap of Yahoo
I think Yahoo is in a deathtrap now. Most of their residual value is in email, and email has gone to the spammers. Yahoo can't afford to fix the problems, and as the value of Yahoo email continues to decline, so does Yahoo's value. Another way to think of it is like an underwater mortgage...
There might be a solution, but I'm convinced Yahoo no longer has the resources or competence to implement it. Hint: They would need to INCREASE the value of email by REDUCING the spam. My suggestion would essentially be a kind of neighborhood watch against spammers, though I admit that vigilante justice sounds better. (Of course it wouldn't actually work that way, because Yahoo would be in the middle of the loop and retaining control.)
Imagine an anti-spam solution similar to SpamCop, but on steroids. Rather than a single confirmation targeting a spammer's ISP and webhosts, it would be a multi-pass analysis with gradual refinement and targeting ALL of the spammers' accomplices and breaking ALL of the spammers' infrastructure and helping ALL of the spammers' victims. You don't have to volunteer to help. I would, and I think there are some other people out there who also hate spam, and all I think we really need is more spam haters than suckers who feed the spammers.
Actually, that raises an interesting new question: How many suckers does it take to sustain each spammer? The ratio of spam to sucker is obviously so enormous that it's hard to conceive of, but the ratio of spam-feeder to spammer must be much lower. It actually seems from the recent spam trends that some spammers are deliberately targeting larger fish, as in religious fools who can be conned by pseudo-charity appeals.