Remember... if Disney is gonna sell this $15 option to the majority of the guests... Disney does NOT want the situation where the majority of guests pay, then get frustrated they didn't get any use out of it because there was no LL availability. That is worst case scenario for Disney.
Rather than open all availability at once and let it be depleted, Disney will be motivated to use more rolling availability. Remember, Disney controls the flow of information to you via MDE/Genie and it's signboards. Through the magic of push, Disney can let you know that XYZ ride now has availability an hour from now... do you want in?? And not only that, Disney can manipulate who it tells when... based on your inputs and past actions.
This is not going to be the scenario like boarding groups where everything is booked out in 1 minute. Disney only serves to LOSE in those kinds of situations. If the product can't return value - Disney will have a hard time selling it. So it's in Disney's interest to keep LL availability open more frequently.