In defense of Chip Kelly, check out the offensive rankings of the NFL teams he was the head coach of.
Seasons of 2nd, 5th, 12th, and 31st overall offensively. The common thread is that he has NEVER led a bad rushing offense in the NFL. For better or for worse, he sticks to a physical brand of football which would seemingly mitigate some of the issues caused by an insane offensive tempo in his earlier years.
Looking at 2013 in particular here, this is the season where Nick Foles threw for 27 TDs and 2 INTs in a partial season under Kelly's playcalling, and this was with absolutely terrible talent at receiver. They were the best rushing team in the league that year behind Shady McCoy, who had the best year of his career in that, his fifth, season. 4th in amount of rushing attempts, but 1st in yards per rush. Vick was injured and did not contribute much, running for 300 yards on the year.
Look especially at the passing efficiency. 27th in total pass attempts yet still 9th best in yards and 5th most passing touchdowns, and that's with a backup quarterback playing most of that in Nick Foles!
2014 saw them maintain a good rushing offense. LeSean McCoy turned in what'd end up being his second most productive year. The Eagles were top ten in both passing attempts and rushing attempts, a function of their tempo. Efficiency did dip a bit, but this is STILL above-average offensive play (outside of the turnovers in the passing game) in both the passing and rushing attacks. This is with, again, below-average receiving talent outside of Jeremy Maclin. The quarterbacks were Nick Foles and Mark freakin' Sanchez.
Things fall off a bit in 2015, but the passing offense is better than it really should be with Sam Bradford and Mark Sanchez shouldering the load at quarterback and the rushing offense, while a bit less efficient without Shady McCoy on the roster, still is functional. Chip is fired right before the season ends at a 6-9 record, having won 10 games each of the previous years.
In 2016, he's saddled with the 49ers after York, Baalke, and Tomsula drove the team into the ground. He immediately fixes their rushing offense and turns it into a top attack. He's saddled with one of the worst rosters I've ever seen in professional sports. Chip gets mediocre yet safe production out of Colin Kaepernick, who tosses 16 TDs to 4 INTs but with a terrible YPA. The receiving corps is legitimately possibly the worst I've ever seen assembled in professional football, and the defense is last in every major category. He's fired after one 2 win year, but he had absolutely no shot in SF if they weren't willing to give it a few years. That roster was absolutely hopelessly putrid.
Chip Kelly has never had an offense underperform its talent level in the NFL, and arguably, his worst traits are those related to GM duties. What I see with Chip is a guy who'll come in, immediately maximize both our stud backs, mitigate our line issues, and do enough to keep the passing offense in decent shape.
Take away the rest of the responsibilities related to head coaching and just let this dude call plays and implement the scheme, and we're honestly probably rolling. He's never had even remotely the level of skill position talent that we do.