I'm going to focus on the bolded line.
Almost all teams have a few players that weren't worth their money. Some are more than $9M, some are just less. Check
https://overthecap.com/ or for the 2019 check out
https://www.spotrac.com/. I'd bet over 1/2 the league have some cap stinkers.
For my Eagles, who under Howie Roseman's guidance, is regareded as a cap smart team, I have no idea why Nelson Agholor occupied $9.3M of cap space just to drop footballs. Yikes! I can almost guarantee Alshon Jeffery won't be worth $15.7M in cap space he'll be occupying next season.
Let's look at the best run dynasty in the Patriots. I realize Brady has done so much and moving from him is hard, but was he worth $21.5M in cap space? His QB rating was worse than the league average. Looking into 2020 do you really think Mohamed Sanu will be worth $6.5M in cap space? Maybe, but he's been a #2 at best for every team he's played on.
Looking at some playoff teams:
Ravens - $15M is a lot to pay for 49 tackles, 2 sacks and 2 ints... (Earl Thomas)
Bills - Paying a center over $11M per year and he gets hurt (Mitch Morse)
Packers - Jimmy Graham's cap his it $11.67M.
Texans - Was Will Fuller worth $10.16M? No.
Chiefs - Take your pick between Frank Clark's $22.7M or Sammy Watkins $21M
Vikings - Could go with Kirk, but Xavier Rhodes has been horrible and his $12.9M cap number makes it that much worse.
49ers - You can look at Dee Ford, Kwon Alexander or Solomon Thomas.
Seahawks - Team has went through a post-boom salary dump, but Duane Brown getting more than $12M is a bit much even before the injury. You could also go with L.J. Collier who was their $3M first rounder who was a healthy scratch this past week.
Titans - Malcolm Butler at $13.81M or Adam Humphries at $9.875M. Take your pick.
Anyway, every team in every sport has player contracts that fall short of expectations. It's almost impossible for a team to win on every single deal. The NFL is unique in that it doesn't fully guarantee contracts (except Cousins), but it's not realistic to expect every under-performer to be cut.