He wasn't obligated to take the Seattle job and there are still other candidates. It's not like he pulled a Josh McDaniels and said he'd do it, get in the building and then at the last second turn back and say "no thanks". He made no promise or, as far as we know, was offered no deal.
That's like calling Ben Johnson a loser last year for not taking a HC job and going back to Detroit to be their OC again, how is that any different from what Frailey is doing? Continuing to build off the success of an already successful team looks better on him and perhaps he has huge loyalty to Dan Campbell, wouldn't loyalty be an admirable trait? Or he felt to himself that he wasn't ready and needed playcalling experience and being the Oline coach and run game coordinator would do that. After all, all the talk was pairing him with someone else to be a second OC to help him call plays. Probably thought that was embarrassing or a lack of trust in his own choices.
Regardless to get this heated over a coach, one of many, that is just interviewing for the job and not taking it seems reductive.