Physical prime and overall peak are different. He was lightning against Gatti, after that there was a slight decline in speed and athleticism, but he was making more complex adjustments at welterweight.
I think he could have fought on in his 40's, but he wouldn't be elite and would certainly take a loss. After the Canelo fight I was starting to contemplate the possibility of an extended career, but then Maidana happened. That version of Marcos might have given any version of welterweight Floyd trouble, but there wasn't much room for error. A fight against a Shawn Porter today, with Floyd at 43, wouldn't end well for him.
That was a phenomenal fight from PBF. I watched it again recently - the speed, timing and killer instinct was something I really appreciated from Floyd coming up. Corrales was methodically broken down, and the body shots in that fight were second to Corrales vs Castillo. Floyd Snr predicting the knockout round was great too.