I'm pretty sure that Ahnold has TRT and HRT as well.
It sounds plausible that injuries def play a role, but is that the only major explanation, and are there ways to prevent/slow down this process?
Of course he has. From 45 / 50 age, it's very hard to lift weights naturally. A treatment is almost a necessity (libido, energy, quality of sleep, skin texture, reflexes, brain function, etc). Arnold can afford the best doctors and products.
Biceps become very fragile from the age of 40. My father broke one of his biceps with a wheelbarrow. So I am very careful: no sudden movements, no mixed grip during deadlift, a good warmup, very very light stretching, no barbell curl (only dumbell). I think the best way to keep the biceps in shape, is a relatively light training (10/15 reps). They respond well to the pump. Also, try to keep the shoulders and forearms healthy. Otherwise biceps try to compensate and tend to break.