Personally, I think the Mario/Peppers comparison is kinda lazy.
Mario Williams and Julius are two entirely different kind of athletes. Both of them elite. But Mario Williams never walked onto a Final Four basketball team and did wild **** like this:
Mario and Peppers are the same size and they play the same position. But in terms of shear athleticism, Mario really couldn't ever hold a candle to Peppers. So I don't think we can reasonably project Mario's longevity because of what Peppers has accomplished.
In each their first 10 seasons: Mario played in 145 games, Peppers 152. Peppers had 100 sacks, Mario 96. Mario had 308 tackles, Peppers 385. Mario has never had an interception, Peppers at 7 in his first 10 years.
Mario was borderline useless a lot year. Was it scheme? Was it insubordination? Was it his physical decline?
I don't think we definitively know the answer, to be honest. But wherever the truth lies (and we'll have a pretty good idea by this time next year), I reckon physical decline will be in the mix.