Demanding money for storage space after a decade of "free, unlimited email" is despicable.
I've been using Yahoo for a number of years, and I had made Yahoo mail my main account, simply because of convenience. I've funneled my other emails through it.
However I will be using a different mail service if you decide to go through with demanding money, especially when you're not offering anything extra, just storage space I don't want. After a decade of "unlimited email" most users have accrued a ton of old emails, But you don't offer a practical way to delete them, while saving the few that are still needed.
It's a really cheap shot, and I'm surprised you'd think customers would put up with this money grab.