This is something widespread throughout more than just Yahoo. Where it was clearly some executive, a manager, or designer, who thinks way too highly of themselves wants to decide for the users what's better for them.

Well guess what, genius, you don't know what the users want.
The users know what they want.
As a designer it is your job to provide users with choice.
Up until now, I thought Yahoo's design team had a shred of brain power because unlike Google they kept their legacy email an option.

Idiotic business decisions like this is what ruin entire software companies, and for a less popular company like Yahoo, this could really spell a large drop in usage that frankly you could not afford.

If I was a director reading this, I would highly reconsider the employment status of the unqualified, overpaid degenerate who considered this was an acceptable business practice.