Agreed. This latest incarnation is the worst yet, and, given that there is an opt-out for the "interest-based" ads, there really is no good reason why Yahoo couldn't continue to allow Classic Mail.
As of today I am no longer able to use Classic Mail. I never told the "trap" page to upgrade, but instead tried to log in via links on other pages (which, for a short time, let me continue using Classic). Nevertheless, all attempts to login now yield the awful new UI.
When are tech companies going to learn that people change more slowly than computers? We want our comfortable surroundings, and we hate to be forced into anything unwillingly.
I realize it's just a UI (assuming you have time to wade through the help pages and find the poorly designed "ad manager" in order to opt-out of interest-based ads), but the principle still applies.