So the wait is over, we can test all the myths...
The big myth being "I will HAVE to wake up at 7am"
"All availability will be gone in the early morning."
So...... It's about 9:20am as I write this.
The very good news: If you were sleeping in till 9am, and just now logging in to
Genie+, you would find EVERY attraction at WDW still available. All G+ attractions, all individual purchase attractions. So if you are sleeping in and arriving later, you still have every option available.
That said, not every park and attraction is equal. If you slept it, it will "hurt" a bit in some regards.
Let's go through the parks briefly:
Magic Kingdom: I suspected things would be smoothest at Magic Kingdom due to the huge number of attractions. I was right. EVERY attraction still had morning availability. Individual purchase attractions are still wide open. You can book Mine train for 10am.
Most G+ return times are still in the 9am-11am window. The worst return time is Peter Pan -- at 9:26am, the return time is 11:25. So just under 2 hours. At EVERY other attraction, the return time is under 2 hours away. So if you decided to sleep in until 9:25, it didn't hurt you at all. You can still get any attraction at MK with a good return time.
Animal Kingdom - -- Things going smoothly as well.
Flight of Passage has a 60 minute standby line. But if you want to pay $11, you can ride right now without line, with a 9:30 return time.
G+ return times are all basically "ride now." For example, Safaris has a return time of 9:35 am. The worst return time in Navi, at 9:50am.
Epcot -- Going ok, not quite as smoothly at MK and AK. Of course, the park doesn't officially open until 10.
Earliest G+ still available for Test Track is 1:15 pm.
For individual purchased attractions, Frozen is wide open. But Remy's first availability is 11:45.
So if you slept in until 9:25 then went to book Epcot, you're still in very good shape, though don't expect to get an immediate return time for Remy or Test Track. At the current rate, Test Track might "sell out" by mid to late afternoon. Remy might sell out by evening.
Hollywood Studios -- By far, the worst park for lines. It was also the worst in FP+ days, being limited with 1 tier-1 attraction, which really had to be booked 60 days in advance..
Every ride is still available at 9:20 am, BUT:
Individual attractions: Rise of the Resistance is close to sold out, the earliest available purchase time is now 7:30 pm. (But Runaway Railroad is still wide open for almost the entire day)
Furthermore -- Return time for Millenium Falcon is 3:35 pm, and Slinky Dog is 7:05pm. So if you slept in, you can't get a reservation for those 2 e-tickets until afternoon/evening. And Slinky Dog will likely be gone by afternoon.
ALL other attractions still have morning availability. You could roll out of bed right now, click RNR with a return time of 11:15am. By the time you shower, grab coffee, get over to the park, it will almost be time to redeem that RNR G+.
So what have we learned thus far -- Even at 9:30 am (as I finish writing this post), availability isn't a major issue. Exceptions being primarily Rise of the Resistance and Slinky Dog, where it would behoove you to try to book those 2 earlier.