I don't blame Pace for giving Leno the extension, because like you said it was warranted, he was playing well. However I do blame him for essentially ignoring the position even though Leno had an entire year of very poor performance on tape. His play in 2019 clearly indicated that he was declining rapidly. Yet FA came and went, nothing; Draft came and went, nothing; Cuts came and went, nothing; Trade Deadline came and went, nothing....
Pace is 100% responsible for knowing that Leno was playing poorly and yet he made zero attempts to provide competition for LT. Spriggs was listed as 2nd string, but he was never given the chance until injuries force our hand.....(Nagy's responsibility).
Pace has his fair share of responsibility for the issues with our O-line.