I'm not sure how they come by the figure, given that the Netflix contract is up next year. At which point Disney Play or whatever they will call it will take over. Disney is just not renewing the contract they aren't breaking it. So they aren't losing any revenue, as its unknown what the contract renewal would have been. And its likely that any contract renewal would have been at a lower price point since the content would have been older. This isn't like Cable where Disney had all the power, saying you have to offer ESPN if you want Disney Channel, etc. Netflix has had most of the power over the last couple of year, with its large subscription base.
Also you think there aren't 40 million people that want Disney content, everything they have from SW, Marvel, Pixar, now Fox, and Disney content. Basically just 1/3 of Netflix subscriber base is all Disney needs. That doesn't mean 1/3 of Netflix users have to switch, just that they only need to be 1/3 the size of Netflix to "break even", if those numbers are true. Plus remember that is only one of their streaming offerings. Also I wouldn't be surprised if Disney didn't even bring out classic content from the vault to offer. Stuff that currently isn't even available to purchase or available to watch elsewhere.