The meta of MMA has evolved. Single leg and double leg take down defense is standard, just like having BJJ defense is a standard. That is why people like Khabib had to evolve their take downs by using cage walls and trips.
Khabib was once like GSP. He primarily set up his takedowns using striking, then changed levels for a double/single. That was successful for a period until the Tibau fight in which he got nullified. After that fight you will notice he completely changed his take down strategy. This is in huge part because he joined AKA where he learned to wrestle with Cormier. The rest is history. No one can stop his take downs.
GSP is an athlete not a fighter. He relied on his athleticism to cover up his chin, which at times has been wobbled by the likes of Hendricks, Condit. After getting exposed by an unranked long shot Serra, GSP changed his game to something safe, valuing point fighting over taking any risk.
It took a juiced Hendricks just to be on the same level as GSP. If a juiced fighter can barely keep up with GSP, it really speaks volumes on how much GSP trains. He has the muscle definition, size and endurance that you don't see in many people who can fight for long periods of time.
GSP's success relied in part because of his timing/explosiveness/technique. If we were to make a pie chart of how much each has contributed to his wins it would look like this.
Given that explosiveness and timing are largely dependent on age, it would be irrefutable to assume that GSP maintained his peak physical form after that many years. This is further supported by GSP slow and sluggish performance against Bisping who was a man already set on retirement while training for GSP.
Khabib will be able to easily stuff GSP's archaic takedowns and demonstrate just how far MMA wrestling has come, by showing techniques that were never thought possible by GSP or any of his past opponents.