Volk is compromised to a degree from mismanaging the last few years of his career, developing a drinking problem and age. Still, with that in mind, he was doing well offensively, but you could see a slight dip in spacial awareness and was a bit more flat footed. That said, the punch he ate was probably putting him out regardless, it just may have never landed that flush if Volkanovski was younger; not coming off of an injury; a brutal knockout loss; and surgery months ago.
Thing is, that's what happens when you're ten to twelve years deep into your career, coming off a brutal loss and are 35, fighting top competition at the lower weight classes. GSP is considered one of the greats and his career was winding down in '03 after almost twelve years of fighting, prompting him to retire. Condit's descent began in '14 after starting in '02 when he got injured against Woodley. Aldo was a few months shy of his twelfth year in MMA at 29 when he began to lose. Fedor started taking losses almost eleven years in. Edgar wasn't the same thirteen years in. Big Nog eleven years. Liddell hit the wall around that time.
Yes, there are outliers like Mighty Mouse, Jon Jones and Anderson Silva (wink wink), but for the most part, the majority of top fighters experience a decline after this time frame and begin to taste defeat more often as reflexes change; wear takes it toll; chins fade; injuries linger; weight cuts get harder.
Topuria was a better athlete and hits hard as shit. History tells us he should beat Volkanovski. He did. And it doesn't lessen the win one bit. Volkanovski did what greats do - dominate a division until they don't and everything catches up to you. No excuse. Just reality. Props to Ilia. He hit Volk with a fucking rocket.