Not at all debateable. He could not get past a L tackle for a long time. And it weakened the defense considerably. It was why I was happy to see him move to playing over center. He could still cause some disruption there.
I think you're cross conflating. He signed his extension in 2013. In 2012, he had 13 sacks in 12 games. In 2013, he had 7.5 in 11 games. In 2014, he had 11 sacks in 16, but spent the season at ILB after the bye week. In 2015, he was "exclusively" an ILB, though they moved him around in dime situations.
His last year with us, 2018, was a legitimate down year. 16 games, only 3.5 sacks, and we let the contract expire.
So yes, I heartily dispute your "get past a L tackle for a long time" statement. Particularly since 2016 and 2017, Peppers was here. I think, but I'm not sure, Peppers played against more LTs...but Pro Football Reference has them both listed as LEFT OLB, so I cannot confirm that at this time.