ExCubes actually become quite common once you hit lv50. Your Daily ARKS Missions give 1 ExCube per character per day, and 10* Weapons drop often enough that you have a steady stream of them from any SH UQ to turn into ExCubes in the Swap Shop menu. I think I got like 15 or 20 just from random drops from today's Wind and Rain.
EDIT: Oh, also, you can use Photon Boosters from AQs to get ExCubes by buying the 11* Weapons from the Photon Booster Exchange and turning those into ExCubes in the Swap Shop.
Yea but you forget to mention, not everyone has time to grind and farm like that. Those things you mentioned take hours to get.