Virtually nobody who knows anything about cycling dismisses Merckx as the GOAT. Likewise, anyone who knows anything about cycling understands that Merckx and Cavendish represent two different skillsets. When I see people calling Cav the GOAT, I understand that to mean he is the GOAT at what he does: fieldsprinting and winning the maximum number of stage-race stages within the stage-race format of his era.
What Merckx accomplished was unprecedented and continues to be unmatched. Likewise, what Cavendish has done is also unprecedented. People were already calling Cav the greatest sprinter of all time 14 years ago.
Cavendish has defeated three generations of sprinters. He put away the Zabel/Freire generation, trudged through and outlasted the Kittel/Greipel/Sagan generation, and continues to defeat the Phillipsen/Merlier generation. He has done this while being counted out over and over again. People act like field sprinting is basically being the first over the line at the end of a group ride. The number of riders who have even competed in as many tours as Cav is very small. For me, the thought of even finishing the stage, much less continuing the Tour (and winning stages!) after the kind of misery he endured in stage 1 is unthinkable. Winning a field sprint requires getting to the end of a stage, navigating absolute chaos, and having the speed, tactics, intuition, and bravery to get across the line first. Today's win was a really good one too.
I don't understand diminishing the accomplishment, especially since I don't think very many people are arguing that Cavendish is "better" than Merckx.