How is doomsday not on its last legs? It is barely open, and when it is, it runs at a third of its power because if it doesn't it shits itself. Once flash & wizard of oz open and scooby re opens that thing is for sure gone, with the strip down and repaint deadline quickly approaching. Only reason they are keeping it around now is to keep the ride count up while other things around the park are being built or renovated.
Lantern is time limited as well. I think it lasts a little longer than 3-5 years, I'd say it goes either shortly before or shortly after the next major gets built in 6-7 years time. As for superman I think village will be looking very closely at how the two LSM conversions go, if they are reliable i think it's more likely it gets that revamp over being removed altogether.
As for the showcase building the park has no room to expand so at the end of the day they'll use whatever space is available. The land doomsday & showcase occupy lend themselves well to a major wild west coaster to complement West, and any non superhero ride is always a plus. Lantern's plot is too small as it stands and doesn't allow for much. As for the other attractions you mention:
Walkthrough? The old intencity building exists (you know where there used to be one)
Dark ride? Justice league (and the junior driving school building exists too), you can arguably fit scooby in this category while you're there.
Show/theatre? Stunt driver and roxy already fill that need