Make it free. Like, "for real" free
I respectfully suggest that you return to your core value proposition as an ad supported free account without size limits. You're never going to be able to compete for features with other providers and all you're doing is alienating your existing customers. Stop trying to make "fetch" happen.
I have no need of new features for my Yahoo account. Your main value to me, as I expect is true of many of your users, is that I've had this account for over 25 years and it serves as an easy index of past emails. Therefore, there's no value add when all of a sudden you make the decision to set size limits and require me to pay to continue to receive email. I'm now just paying for what I used to get for free or you're holding my account *******. Offering the option to download my data is just passing the burden onto the consumer, especially as I have no idea if I'd even be able to make use of the data once I downloaded it.