Nobody outside of Mexico and hardcore boxing fans knew who Canelo was when he first fought Floyd. Now he's a much bigger star. And it'd be sold on him being much more dangerous vs an over-the-hill Floyd.
GGG isn't a big PPV draw, but he can be hyped as a potential "Mayweather-Killer". People pay to see Mayweather lose, and if GGG is correctly marketed as a guy that's too strong and dangerous for Mayweather, then it's a huge, huge fight. The media would turn it into a superfight and hype GGG like he was Mike Tyson, and the public would buy it.
Today, both Canelo and GGG would do massive numbers if matched against Floyd, as both would be hyped as the ones capable of handing Floyd his first L.