As I'm still trying to understand all of this angst - here's my question - will the 9-10 hour now be so much more crowded? Will people who were planning to sleep in and arrive at 10 now change that and arrive at 9?
My gut feeling is no. Our wake up times depend very much on what we did the day before and how long we plan on staying at the park that day.
To those canceling ADR's because their plan counts on rope drop - I am not expert, but I think that is probably a little over the top. I don't think the crowds you will experience from 9-10 now will be significantly different than the crowds you would have experienced before.
My angst is that I spend so much time researching to maximize what my family wants from it's WDW trip (which may not be what you want) and if being there as soon as the park opened was key to maximizing our fun, then changing park opening changes the value of my family's vacation. So it's a change in value for $ and time spent, which can be incredibly frustrating.
If someone was going to sleep 'til 10 they will probably sleep 'til 10. Our experience, however, has been that very many people try to be up for RD regardless of time but a large percentage just don't get out the door early enough; thus, first hour is important regardless of the timing of that first hour. We are still hoping, though, that the sleep 'til 10 folks are a significant part of December touring.

Sometimes the time in the park is >> than the special ADR and sometimes the ADR is most important. I think the problem comes from both being about equally important and going from "having it all" to having to choose when that choice is not easy and when you, as a parent, thought you had planned the ultimate trip for your family.
So we are going to enjoy our first Christmas at WDW and have done some minor shifting of plans only. But I have (1) teenagers who are better able to cope with touring hours, endurance, and rolling with the changes; (2) the luxury of having been so many times that we don't worry about missing something this trip; (3) no early breakfast ADRs; and (4) we knew this was more a holiday trip than a WDW trip, so holiday atmosphere is where we are expecting our value this trip. All I have to decide is how early am I willing to get up for less crowded parks, but others have a tougher choice.
I think the typical family going to WDW in December is trying to plan the ultimate WDW trip for the kids while enjoying the holiday atmosphere - again, "having it all," which looked possible with "originally published" park hours and had been easy in recent previous years. I've been there, done that, and (absolutely

) bought the T-shirt. I know Disney needs to spread out the (higher than ever) crowds, so they specifically can't let anyone "have it all" when it needs to be spread around; but it's hard to accept, when you had it all scheduled
perfectly for your family, that now you must choose to give something up because of a change that was out of your control. Sympathies and

to all who are trying to make tough choices to maximize their family fun with the changes.