As most have mentioned, they were hardly a playoff team. They overachieved big time last season and even in week 17 David Fales almost led a comeback that would have rendered the Bengals miracle moot anyway.
Last off-season McDermott added some pieces to his defense in free agency to help with a transition period (Hyde, Poyer) while relying on Kyle Williams and Zo. The offense was steady enough with the run game and a strong OL and a QB who wouldn't lose you the game (but also never won it).
What is happening this season was suppose to happen last season after the trade of Watkins and other key players. This season it is magnified because they made the playoffs and drafted Josh Allen in the first round.
Losing Wood and Incognito on the line really did damage to the offense. Not to mention they still have no receiving weapons (they didn't last year either but this season its even worse).
Defensively they just aren't getting the turnovers that they were last year at this time and Edmunds has been drastically inconsistent which has not helped.
This year is going to be a long one but hopefully the young players at key positions getting a lot of playing time can grow and develop and after an off-season with 60+ mil in cap space and 10 draft picks they can return to a 6-8 win team in 2019. Followed by hopefully a real playoff run in 2020 depending how they players have developed. IMo McDermott isn't going anywhere soon with 4 years remaining on his contract.