Who says it was Yahoo's decision?
Is it not possible that they got an order from a three-letter agency to make migration as difficult as possible?
If the other email providers are playing hardball with the Government (doubtful, I grant you) then maybe they're just trying to close this particular cage before all the rats escape?
I know this all sounds a bit tinfoilhattish, but I'd say that is a reflection of the zeitgeist. 2016 has been weird.