I didn't have an issue with the players he let go, McCoy was way over paid and a headcase in the locker room, Maclin is oft-injured and a decent number two, but got elite level WR money, Foles was a disaster and Jackson was on the heels of the Hernandez incident and had his CA gang buddies all over Philadelphia and parts of Jersey.
Kelly's problem was he is a terrible judge of talent at this level. The players he drafted or brought in were mostly horrendous and he seemed to target guys he had familiarity with while recruiting in college and overpaid and/or selected them well higher than their projected value. In addition, he completely disregarded the offensive line, which ultimately destroyed the offense. You can get away with inferior offensive line talent in college, for the most part, if you're loaded at the skill positions, but that doesn't work in the NFL where even the worst of the teams have elite NFL level talent in the front seven, unlike the majority of collegiate opposition.
I think his offensive system could work in the NFL, but would need to be implemented by someone other than Kelly or have him enter a situation where there is already an elite QB and stout offensive line. Unfortunately for him, if those two things are already in place the team is probably having some success and not likely in the market for a head coach.