Rotton core of Yahoo! email = SPAM
My own belief is that Yahoo's #1 problem is a rotten kernel, in this case email as the core of Yahoo's residual business. What's rotten about email? SPAM to the degree where email has lost its value. Actually, Yahoo's filtering is relatively good in terms of false positives. In my experience Gmail has a relatively high false positive rate, so you probably won't lose real email by ignoring the Yahoo spam folder, but Yahoo makes up for it by their support of spammers (as in scam dropboxes) and in their rejection of spam reports (with a labyrinthine and tedious spam-reporting system.
Hey, Yahoo. Do you actually want to survive? If so, why don't you make it so EASY to report spam that the spammers will stop abusing your reputation?
Two things to remember: (1) There are far more people who hate spam than suckers who will send money to the spammers, so even a small percentage of the spam haters could help cut the spammers off from their loot. (2) The spammers can't obfuscate beyond what humans can understand when they are trying to get a human response (and remember that the suckers are way below average intelligence, too).
In conclusion, imagine a spam-fighting system built into Yahoo, something like SpamCop on steroids. Whereas SpamCop just has one round of analysis and confirmation going after the spammers' ISPs and the spammers' webhosts, such a system can and should be strengthened to go after ALL of the spammers' infrastructure and accomplices. Beyond that, it should help the secondary victims (like the slandered or defrauded companies) and even let us know when it is safe to unsubscribe (by testing the unsubscribe mechanisms for address harvesting).
In conclusion, I think Yahoo is earning their imminent bankruptcy. I only hope it inconveniences the spammers more than the rest of us.