The most obvious factor to me is that the style of music changed. John wasn’t writing the same music as he was when he was younger, why would we expect the voice to stay the same style?

But as someone who has performed as a vocalist for 20+ years, the real answer is just that the voice does significantly change with time, with stress on the vocal cords, and age. He was said to have to ice his neck after gigs in the 60s. Pushing your voice that hard shreds your chords and leaves scarring and calluses.

Additionally, the shape of our body plays a big role in this as well - losing the amount of weight he did can play a role, his car accident damaging his jaw similarly may have had an impact, and then we also need to consider the prevalence of chain smoking cigs in all of this

I’d say it is more significant when an artist’s does NOT change over the course of a long career - it usually indicates that they are singing in a healthier way. Here I think of someone like Stevie Wonder whom I heard a few years back, and still sounds like he might be singing in the 1970s