Networks have given time back to local affilliates before though at that time it was government regulated. Until 1970 the then 3 networks programed the 7 PM to 8PM hour. Usually with the network news at 7 PM and a sitcom at 7:30PM. The local stations filled the hour much like they do now with...
I don't think network programming will ever disappear but I believe there will be less scripted programing with more reality and game shows. The cost of scripted shows is just becoming too expensive with viewership continually declining.
Soonest it would happen would be in one year. I could see local stations putting on a one hour local news show at 10 PM and start the Tonight Show at 11PM.
https://deadline.com/2022/08/nbc-could-give-third-hour-of-primetime-back-to-affiliates-1235101078/