Greb lost two "up in the air" decisions that could have gone either way to a guy he beat when he had ONE eye.
In a one belt, 8 division era most fighters won't get the spotlight you are used to seeing. Charlos would be your Mike McTigues (MAYBE). Danny Jacobs would be a Jimmy Slattery (MAYBE). If this were 100 years ago, GGG and Danny would have fought 4 times by now. Canelo wouldn't have clenbuterol, and the long breaks between fights to matchmake. It's the same sport but very different. Tommy Loughran and Tunney would be champs today at LHW, and, if they had modern sports science, they would both be champs, maybe avoiding each other in order to keep undefeated records in this douchebag era. Sam Langford would demolish anyone from 154 to 175 in today's divisions. Today we have 16 divisions and at least 4 major titles in each, several of which have diamond and interim status. This is the reason you are diminishing so many legitimate boxers from over a century ago. They constantly fought without any champ recognition, so even really good fighters would get overlooked by casuals today. If you quadruple the chances that all those names get belts, just going from 1 to 4, then multiply further by doubling weight divisions, you see that there were fewer opportunities for so many of these guys back in the 20's who would be champs by today's standards.
With more belts and divisions you would have more cherry picking, too. Also, fighters fought more back then. You're bound to have an off night. A guy like Floyd would have lost numerous times if he had their schedule and if there was only one belt. It' the way it goes. You will take more losses fighting more, especially with Greb's resume listed above.
One should not apply the 21st century boxing landscape to the 1920's. Not one of today's fighters would have a better resume if they fought in Greb's division or LHW back then.