You can't compare GGG to Floyd. Floyd moved from super featherweight to light middle throughout his career.
The reason GGG should think about moving up is simple. Two of his mandatories are pure garbage and the other is Daniel Jacobs. Jacobs would be half interesting. Failing that Eubank Jr and Saunders are the only things left for him given Canelo's reluctance. Despite Eubank's mouth, he has done very little to suggest he is a realistic threat to GGG. Saunders is clearly of similar standard, so I don't really see the challenge.
So, if he is wanting to chase greatness, he should be challenging himself, and his bigger challenges are at 168. De Gale and Ramirez are both challenges. Even Callum Smith or George Groves would be fairly interesting...at least on par with Eubank and Saunders, with the added threat - with them being naturally bigger. I'd even rather see him in with Abraham or Bute over Saunders to be honest, just to see how he deals with natural weight disadvantage (if it is even apparent). He's certainly not at a debilitating height disadvantage at 168. He's taller than Abraham, an inch shorter than Groves and 1.5 inch shorter than De Gale. Hardly a problem. There's only really Ramirez and Bute who have a few inches on him, but he's expecting Brook to deal with a similar disadvantage so it would be shame for him to be hypocritical about it.
Moving up is certainly less ridiculous than expecting Brook to put in a competitive display against him. Who cares about being an undisputed champion when the division is garbage? It's a sad state of affairs when the supposed p4p champions biggest wins are Lemieux, Rubio and Martin Murray.