You have the plumbers in there working, and then you bring them back. Rather then remove parts and then reattach, it's easier to cut out and replace. It saves time to remove and replace. Fitting new pipe into old pipe takes time.
Electrolysis, Anytime you move between copper and galvanized, you shorten the life of the existing galvanized pipe. Copper and steel are not real compatible.
As you turn off the water and then bring it back on, the end parts of the runs will break out crud from the inside of the pipe. That winds up in the cartridges and the aerators. You will wind up doing some flushing.
Most of the slow down in the pipe will be where the most electrolysis has occurred. That is any place where brass meets steel. That means the shutoffs at the wall have the most reduction. A attic repipe hasn't done anything for those connections.
Tub and shower valves, these are made of brass, same problem there. The steel will corrode and close off where it meets the brass.
Replacing the horizontals may help a little, but also speeds the demise of the pipe that has been left behind.