The facts don't support your statement. In Garrett's 6 years as OC, from 2008 - 2013, he had 6 top 13 ranked offenses, and 4 top 10 ranked offenses. By any reasonable standard, he was a successful OC.
2007 - 3rd
2008 - 13th
2009 - 2nd
2010 - 7th
2011 - 11th
2012 - 6th
Average ranking in those 6 years - 7th
Since 2013, when Garrett stopped calling the plays:
2013 - 16th (Callahan)
2014 - 7th (Linehan)
2015 - 22nd (Linehan)
2016 - 5th (Linehan)
2017 - 14th (LInehan)
Average ranking since Garrett gave up OC job - 13th. And 3 of those seasons ranked worse than ANY season Garrett was calling the plays.
Conclusion: Garrett was (and still could be) a top 10 OC.