Offense looked good early on because Ben was bailing the O out on an unsustainable level on 3rd downs, 3rd and long specifically. There was no way to sustain the numbers he was putting up on those downs, no qb could be that good in those situations for that long of a period before falling back to normal levels. 3rd down success rate has a good bit of variance to it and it's not something your offense should count on to look good.
The best thing for any offense is to avoid getting in those situations in the first place. The O looked bad because it stopped converting an inordinate amount of 3rd and longs.
I'll have no problem if Haley leaves.
Under Arians, pretty sure the Steelers were ranked in the top 10 in offensive DVOA 4 out of 5 years, yet was constantly under fire. Under Haley, they were 19th. His previous year, Haley's O was 29th, 13th the year before that, 25th the year before that, yet he's an offensive guru when he has Kurt Warner, Fitzgerald, Boldin, etc.
Arians's offense never looked great to traditional ratings cause they were a ball control offense and traditional offensive stats in NFL are pretty damn misleading. The Colts with a rookie QB improved by 10 spots in DVOA this year with Arians serving as the head coach practically the whole year. He's obviously a very good coach that yinzers unfairly ragged on.