Here's the issue that I have with your assessment.
1. Maclin and Wallace were having a great deal of success a year to two years ago, tops. Wallace was a 1,000 yard receiver a year ago and Maclin damn well would have been if he were healthy. And Watson was a massively explosive tight end with the Saints. I'm willing to agree that the Achilles probably sapped him of some of that. But to sit here and mention Jason Avant as a supporting argument is really weak. Dude was not that good and was nothing more than a possession receiver. He wasn't a great talent by any stretch and Flacco has definitely had better to work with in his career.
2. Sean McVay was not the play caller in Washington. It was Gruden. But there he is in LA doing one of the best jobs in the entire NFL. The fact that he worked under an offensive genius is a bigger indictment on this offense than anything because that means Marty learned under Reid and learned how to design offenses with Reid.
3. Except, what coordinators really just flame out in a year because they turned 55. Like, BB is over 70 and doing fine. Same for Pete Carroll. This isn't a position that requires peak physical shape and Marty was doing very well very recently for the Eagles, so that should tell you something.
4. Do you watch the All-22? Like, I understand people want to make this claim that the play calling is bad, but never actually watch the All-22 to confirm the play calling is bad.
The claim of Marty's past success was more that he was clearly designing explosive offenses that were targeting the deep parts of the field and were aimed at gaining chunk yardage. The fact that it has gone in a total reverse in execution here is suspicious, to say the least. You don't have the most explosive offenses in the NFL to the worst offense in the NFL by chance. Realistically, I would buy this argument and any argument about the supporting cast if Flacco didn't create so many problems for himself with poor mechanics are horrendous accuracy. Fact of the matter is the Ravens supporting cast favors a deep passing attack by a mile (Watson, Wallace, Maclin, Moore, Perriman) and Joe ranks 2nd to last in deep ball attempts.
Oh, and bringing up Pitta is REALLY weak. Like, I'm not sure what you thought you'd gain from it. Pitta was in TC for like three days. He got nothing from the new offense or what was expected to happen, especially considering that Joe wasn't even in TC. Yeah, I'm sure Pitta really knew how the offense was going to look come September when he was in TC for a few days and never once with Joe. Makes sense.
Both Harbaugh and Joe himself have already debunked the PItta comments as untrue and an attempt to stand up for Flacco (Flacco's words, not mine), so I'm not sure why anyone brings it up.
And to answer your question about the Saints and Chiefs, Pete Carmichael since he won the Super Bowl with them and has been in NO for quite a while and Matt Nagy because he was the quarterbacks coach and actually played at University of Deleware.