"The Scheduled Rides feature allows you to select a 10 minute window for a driver to come pick you up. At the start of the window, the app will send out a ride request on your behalf."
So if you "schedule a ride" at 7pm, it is exactly the same as just using the app manually at 6:50pm. There is absolutely nothing remotely close to a guarantee that anyone will you pick you up at all, much less at the time you chose.
Three days ago I scheduled an Uber for 5:00AM-5:10AM.
It arrived at 4:57AM.
EDIT: Perhaps you misread the description...
"The Scheduled Rides feature allows you to book a trip in advance by selecting a 10-minute pickup window. The driver will be requested on your behalf and will arrive in the 10-minute window you've selected. In the case where a driver is not available, you'll be notified."
https://help.uber.com/riders/article/scheduling-a-ride-in-ad...
There would be little sense to doing what you suggested (queueing rides 10min in advance for scheduled riders) because Uber can use scheduled rides as constraints for pathing of vehicles, and some areas would have a greater than 10 minute wait time for pickup.
Neither Uber or Lyft will even tell you if historically drivers have been available if you request a weird time. In some places you can call an Uber at 5am, in others that'd never work, and figuring out which requires local knowledge which runs contrary to the whole point of Uber/Lyft.
Source: friend who works at Uber who looked into exactly this topic.