We'll just have to agree to disagree on the last part of your first statement. Cousins actually HAD McVay as his OC one year, and choked with a chance to get to the playoffs - and it was largely on him, not the rest of the team. And then he turned around and took over a team that Case Keenum had taken to the NFC Championship and they don't even make the playoffs. One playoff win in his career on some talented teams (and much more talented than Stafford ever had in Detroit, which over the last 5 decades or so is even worse than the Browns). Both a playoff-worthy Redskin team came up short with him, and the Vikings have underachieved as well. You wouldn't excuse Goff in such a situation, so at some point you've got to be consistent and admit that it's Cousins that is the one who's coming up short.
Dak's coaches may not have been HOF level like McVay, but they weren't terrible either and he had a ton of talent around him. As dieter mentioned, he had the NFL rushing leader for 3 seasons, not to mention good WRs and the horses up front. Dak hasn't been able to make the plays when he needed to.
Mayfield had plenty of talent around him in Cleveland - OBJ, Landry and a good RB in Chubb. What did he do with it? Not enough to even get a 2nd contract and also only one playoff win and one playoff appearance. Mayfield has trouble making the plays even in the regular season.
Goff certainly had his struggles in his final two years with the Rams, but I can't see Dak, Cousins, or Mayfield doing any better than he did put in the exact same situation. Dak would have been the best of that bunch, but I can see Cousins and Mayfield shitting the bed much worse than Goff did because they have shown they can shit the bed with even more talent around them and with decent coaches and coordinators.