That's the key there. If they were saying something like "Pay now, before we put the movie back in the old "Disney Vault" and you'll never see it again" that's one thing.
Disney Vault - Wikipedia
Any one who chooses to pay for it is just paying to see it earlier. Lots of movies are delayed. If you want to see the movie, but you don't want to pay extra, just imagine that it was delayed a couple of months and you get what you want. It's not like this is the type of movie that you NEED to see on day one in order to avoid spoilers. It's a remake. The vast majority of the audience knows the plot already.
Having said that, I SEE what Disney is doing.
I'm not paying for Disney+ because I'm still on my 1 year free trial that I got from buying an LG TV last December. I'm sure a large percentage of people on Disney+ are still under similar free trials and not paying for it. By delaying the movie, what they're doing is waiting out some of us "Freeloaders". So the cost of streaming the movie makes up for the fact that a lot of subscribers aren't paying anything at all. By the time it comes out for free in December, the 1 year free trials will be expiring and most of the people watching it will be people actually paying for their subscriptions.