Yeah, but what about Lizard and Spock? (TBBT reference, lol)
GSP was a well-rounded a mixed martial artist as you could possibly get, but that isn't what made him effective at neutralizing his oppnents strengths or exploiting their weaknesses. It was his discipline, his ability to listen to his corner, and to actually follow a gameplan. His fight IQ was good enough to do as he was told, but aware enough to recognize and adapt when necessary.
To the original point, I don't think Volk vs Topuria really came down to a fighter being more or less well-rounded or one of the two being a better specialist (either offensively or defensively).
Ultimately, I think it came down to factors that were simply beyond Volk's control. The most notable factor was the speed and there is nothing that Volk can do to gain the edge in that department. Ilia was simply faster, it just seemed like Volk was in first gear and Ilia was in second. The other thing I noticed was that Ilia was better at gauging the range and getting in and out. Again, at 35 years old, it's a factor that Volk really can't improve on to the point where he becomes better than Ilia at.
I believe if he really wanted to, Topuria could probably put together a pretty lengthy reign at FW. Something tells me he won't though. He just seems the type driven by the challenge and the fight, not by the belt. If he rematches Volk to the same end, and beats Holloway (who I would expect to be his next opponent), hanging around 145 for another contender to emerge seems like a waste of time and talent.