Pipe length confusions when replacing tub/shower diverter
How do you know the diverter is "hopelessly" scaled up? Unscrew or otherwise detach the spout from the supply line and soak the diverter end of the spout in vinegar or in CLR to descale it. Then reattach it to the pipe.

From my understanding of your situation you do not have to change this pipe. Hard copper tubing with sweated on end fittings is better performing, but unless your pipe is leaking you don't absolutely have to change it. This pipe does not contain water when the valves are shut. So you cannot have a catastrophic failure of the plastic pipe when you are not using it.