Well any offense is going to look good with great players but bear in mind that offense also worked when Arians was OC of the Colts and when he was with the Steelers.
Nagy had only called plays for 6 games as he took over for the Jets game in 2017. Leftwich has been calling plays for a few years now. That is a world of difference in the NFL. Teams were still trying to figure Nagy when he was hired. They seemed to figure his O out sometime in 2018 and he has never adjusted. By contrast teams have already had like 40 plus games of Leftwich as a playcaller so plenty of time for them to have figured him out and for him to have adjusted to it.
So, the offense worked when he had an in his prime Ben Roethlisberger and pre-damaged Andrew Luck. I am not saying Arians is not a good coach or that his system can't work. I would have loved it if Chicago had hired him instead of Trestman. However, like many, his success is directly tied to when he has outstanding qb play - Big Ben, Luck, a resurgent Carson Palmer for a few years, and now Tom Brady. He hasn't exactly had to make chicken salad out of chicken crap at his qb position very often.
My concern with Leftwich is just overall experience. I don't like that he is only in his 5th year as an NFL coach, 6 if you want to count his intern year. I don't like that he has only been a qb guy or offensive coordinator and that basically all his time has been spent under Arians and the Arians system. Been there, done that, with Nagy and the Reid system. I would like Leftwich a lot better if he had just a couple more years experience and away from Arians and his system.
You look at guys like Sean McVay or Kyle Shanahan who were offensive gurus. McVay spent 9 years as an assistant before becoming a head coach - and it wasn't just qb's. He coached WR and TE before being an OC and under two different head coaches (Mike Shanahan and Jay Gruden). Shanahan spent 13 years as an assistant on multiple teams with different head coaches. He coached wide receivers, qb's and then spent 9 years an OC before becoming a head coach.
I think it's beneficial to hire a guy, whether he a first time head coach or a retread, that has more just a handful years of coaching experience and in different regimes. I want the guy to have seen and experienced different styles of how to coach, how to adapt, and how to lead. I just think a stronger background like that will serve the person better to find success in Chicago.