Would the original water system manufacturer refuse to service the system or void the warranty when a generic cartridge is used?
A replacement cartridge is a consumable, not under the system warranty. As explained in Question 1, a replacement cartridge is not likely a cause of any system problems. Thus the original manufacturer is not likely to void the warranty. It is illegal to refuse a reasonable service request under warranty in many countries. There are many similar cases where replacement consumables are generic, such as ink cartridges in printers, cartridges and columns in HPLC systems, etc.