Here's my take:
First, I completely agree about sub2. Now, on to the substance:
GOAT means the 'greatest' - it's a comparative measure. Even when the competition/talent pool is weak, there is still a 'greatest.' For example, I'm sure there's a 'greatest' curler, or dart thrower, or zamboni driver, and the list goes on.
So, who is the greatest marathoner? You admitted in your second post that a world record is not enough to be the greatest. Kimetto is great and has the greatest marathon time, but he's not the greatest marathoner. So, who is? Kipchoge has done many great things at the marathon distance, as in many wins at major marathons such as London, Berlin, and Chicago. He has olympic gold. He has won those races with style and dominance. He has a very fast PB. (He's one of two people to have beaten Rupp, which I add for the Rupp haters).
If Kipchoge is not the greatest, you're probably going to have to go with somebody who is now retired, who also doesn't hold the world record. My point is that there are several candidates for GOAT marathoner. Kimetto is great but probably not the greatest. Kipchoge is definitely a legitimate candidate, as are several from years gone by. Bill Rodgers has so many wins. Abebe Bikila won with panache at two olympics. Don't forget the likes of Grete Waitz and Ingrid Kristiansen either!
It would be more interesting if you tell us who the greatest is and then we can flame on!