There is the transfer at 7th Avenue to the which would become even more attractive with increased frequencies. there goes your backtracking problem.
Having the 6th Avenue run local would increase demand on 53rd Street even more as it would eliminate all incentives for riders to stay on the Queens Boulevard local (the ).
The only time the locals were extended past Forest Hills were the original EE from December 31, 1936 to December 15, 1940 during off-peak hours, the from December 11, 1988 to September 30, 1990 at all times and then during rush hours to October 26, 1992, and the late night from September 30, 1990 to August 30, 1997. There were only complaints with the . Remember that 77% of passengers benefitted from express/ local to 179th. The change that cut back service increased travel time along the by 3.5 minutes.
With deinterlining, like on @RR503's second deinterlining map, you can have some Queens Boulevard express trains go to Jamaica Center (12 TPH), and have the rest go to 179th Street, with local stops covered by the .