Hello: Water heaters like being taken care of! Yours is testament to the fact. About your brother’s heaters: Has he changed anodes as well and at the right times? Is water pressure not too high or variable?
More water use does seem to affect anode life and it makes heaters work harder, but I’ve yet to see credible research that can connect normal vs low usage to heater longevity in heaters that have been maintained. Most studies use accelerated testing, which doesn’t capture what we’re talking about. Your theory about cracked glass might hold some truth, but the anode still should prevent rusting at places in the tank with damaged glass.
Water heaters have been made from copper and monel. Both metals last for many years. I’ve been fooling with heater maintenance a long time and I get the “you must be from Mars” look a lot. Welcome to the club! 😎