NFL opening day! Ravens vs Chiefs

It's both though. You need a QB like Mahomes to really succeed with Reid's scheme. He had all the pieces in Philly but couldn't quite seal the deal. With even more talent and a QB like Mahomes, he's getting even better results. So it really goes hand in hand.
What were all of those pieces in Philly? During his 4 straight NFC run, the only season McNabb had any pass catcher on the level of Kelce was his time with TO. That was McNabb's best season as a QB and Andy Reid's best season in Philly. People really don't remember how weak those receivers were that McNabb had for most of his time in Philly. That's why I said that people don't realize that Mahomes have been on the best teams that Andy Reid has had. I would have put an asterisk on that though, the team that he began building at the end of his time in Philly would have been the best but he had all of that personal stuff going on at the same time.

Belichick needed Brady to have the success he did.
The legend of Matt Cassell in the NFL began with an 11 win season.

When Brady left, the Patriots pretty much fell apart. If Reid goes anywhere else, or Mahomes goes anywhere else, Reid isn't likely to be able to continue have the level of success he's had with Mahomes.
In the same vein, would Mahomes have the same level of success outside of Reid and his system. Using Brady as an example, he was in the same offense throughout his entire time in New England. It evolved but the offense was the offense. When he went to Tampa, he struggled running Bruce Arians system and they ultimately had to modify it for him and the next season, the were basically running what he ran in . Peyton Manning brought the exact same offense to Denver than he ran in Indy.