Except you’d have a lot of the same problems you have now.
Neither FastPass, FastPass+ nor Genie+ are skip the line systems.
None of these change the core of how the system works operationally. Ever closed gift shop also diminishes capacity. Every closed cashier at a quick service dining venue diminishes capacity. The problems with Genie+ are not because of these front end modifications to make a system work with the current constraints, they’re fundamental problems with virtual queue/reservation systems that Disney has been able to ignore and put off addressing.
FastPass+ was about capacity, pushing people to underutilized corners to avoid having to build more. It was other aspects of NextGen that were supposed to get you to just spend more because you so enamored with the technology (MagicBand), Cast Members knowing your plans (killed for being creepy) and data driven marketing (killed for being creepy and a legal nightmare).