I think this whole Cardio thing is overblown to a degree, he's seen to get a second wind in the Nate Diaz rematch, and didn't look like shit in the 3rd round of the Khabib fight, he even "Won" it according to the Judges that night and defended takedowns. The only thing Conor loses is that sting in his punches, he doesn't automatically become a lesser fighter. Yeah his pullback style of counter striking doesn't have that same speed, but he still has his wits about him and can string combinations and push the pace if need be.
Conor's problem, is how he mentally wilts when fatigued and can be easily controlled at times. With Nate Diaz he was able to use his size, and get him against the cage and work that Dirty Boxing style of fight, with Khabib, same thing except with a dangerous and layered grappling threat.
His Cardio or lack thereof is only a detriment when he's faced with a style that takes advantage of that and smothers him. Khabib will always be able to grapple Fighters against the cage and take them to school and even Masvidal found himself pushed against the cage against Nate, these two fighters have styles that are BITCH to ANYONE without elite level endurance, it's not just Conor, and not many fighters will be able to "Bully" him like they did. Masvidal was also throwing everything and the kitchen sink and Nate and he kept coming.
If anyone watches the 4th round of the 2nd Diaz fight, you'll see Conor was still in it, he disengaged multiple clinch attempts out in the open and defended a few takedowns against the cage, but Diaz was still able to make it a tough fight because he's got a Chin from hell. Nobody else that stood with Conor for more than 2 rounds didn't get TKO'ed besides Max Holloway YEARS ago. Khabib smashed the fuck out of him, we all know that much.