With his young age, he will have long time to be able to maintain it, although there are also some young athletes who have to suffer and accept fatal consequences to the point of risking their lives due to the use of steroids in high doses.
I am not as optimistic as you are. The problem he has in the kidney, an autoimmune disease, is probably controlled for the time being but the damage to the kidneys does not regenerate, it does not happen as with the liver, and the combination of steroids and a diet very high in protein sure does not help.
Add to this that the life expectancy of professional bodybuilders is greatly reduced because of this, and people like Schwarzenegger who reach old age are the exception rather than the rule, plus in his time they didn't take the huge amounts of steroids that they do now, you see people in local gyms who are bigger than him in his prime.
You are right, it's impossible for kidneys to repair, once there is a damage to them, that's permanent but liver on the other hands, repairs very easily. I can't believe my eyes that Chris Bumstead is a bodybuilder, he has an IgA Nephropathy, which makes it harder for the kidney to filter waste from the blood. He was in such a bad condition that he was hospitalized, so I have no idea how he was reaching limits every year with that condition, without getting hospitalized. He should be on a very high protein diet to maintain muscle but to be honest, he isn't big guy off-season, I have seen images of him where he looks pretty average. I have a sense that he takes care of his health, at least I get that from his speech but his high protein diet doesn't speak so and I can't believe how he is a 6th times Mr Olympia in a row, that's amazing.
By the way, in any way, we can say that this guy is genetically gifted. He looks like a Hollywood star and his physique is out of this world. Truly a genetic FREAK.