Where top level MMA fighters tend to really shine over less experienced fighters is being able to mix multiple phases of fighting together - transitioning from grappling to striking, wrestling straight into subs, sweeps to standing up and striking again immediately. This isn't an easy thing to learn and generally takes years and many fights to do it - you can train kickboxing or BJJ or wrestling and if you are a freak pick it up fairly quickly (not one year quickly, but much faster than most people); but being able to integrate them together takes a lot of time and experience - generally it takes a lot of fights in MMA to do it. Not unlike building up the ability to go 12 rounds as a boxer at high intensity, which typically takes years, even for talented boxers young in their careers. This is what allows MMA fighters to land strikes against elite strikers that would destroy them in a kickboxing match, or land takedowns on high level wrestlers who would ragdoll them in a pure wrestling match.
As a separate issue, I have no doubt that there are boxers CAPABLE of becoming elite MMA fighters with years of work. But whereas a #3 ranked boxer might make it into the top levels of MMA, a #1 ranked boxer might not even come close - you see this in specialists from other disciplines coming to MMA all the time. There are high level Judokas who were competitive in the UFC, and ones who get throttled in smaller orgs; same with kickboxers, wrestlers, BJJ players and so on. This is particularly true with a super-narrow sport like boxing.
Bro...Other than Fedor/Yoel and a very few guys...Nobody mixes the multiple phases of fighting in a natural manner.
I seen MMA a lot, nobody has that flawless transition, other than Fedor/Yoel, perhaps I missing some other names but it's really fucking rare.
Conor is certainly not one of those guys who mixes his striking perfectly with his takedowns...I seen his striking to takedowns, they don't look as if it was one move, they look very mechanical...
Again Im gonna bring up Nate Diaz...Although he is a BJJ Blackbelt, he is just a boxer in MMA...Seriously, he is flat footed, slow, and just keeps walking straight, he does basically zero transitions except for the very occasional takedown....and I seen his striking to takedown, nothing special about it...Nothing that will take years to learn...Sorry, if Nate Diaz can do it ,Canelo/GGG can do it.
MMA Fighters land strikes against kickboxers, because A) Kickboxers Defense sucks, it really does and B) I seen Kickboxers vs Wrestlers, they focus too much on the takedowns so get hit.
Again Im not saying Canelo/GGG will become the GOAT MMA fighters...Im simply talking about Conor vs GGG/Canelo......Khabib/Ferguson/etc would destroy GGG/Canelo because like you said, it takes years to stop a legit wrestlers takedown.
Conor is not a wrestler...His wrestling shot sucks ass...it wouldn't take much to defend one of his takedowns.
Nate Diaz taught us that Conor's weakness is boxing.....Canelo/GGG are 10x better, more powerful, and more mobile than Nate Diaz.
Canelo/GGG are simply a bad matchup for Conor in MMA.....I honestly think that Canelo/GGG have the body type/Athleticism/Size to have way better TDD than Nate Diaz or anybody Manlet featherweight that Conor has taken down.
All Canelo/GGG have to do is train TDD/Learn to be aware of kicks/Check leg kicks(Nate Diaz biggest weakness that he learned a couple of fights ago and he got far in MMA)/defensive BJJ.
Again, Im not saying they will be the best MMA fighters ever(They will need years to be well rounded enough to beat everybody,, and like you said it will depend on the Fighter because being well rounded is not easy for everybody)...but they will certainly be a horrible match up for Conor.