Absent Jerry, Jimmy Johnson was goiung to coach for someone else, perhaps even the Cowboys under Tex Schram, although I doubt it.
Simply put, Jimmy Johnson had a specific plan on how he was going to build the team and was going to coach for the team that gave him the most freedom with least interference to implement that plan.
That team was the Cowboys.....until they started winning. Suddenly, Jerry Jones, who previously took the GM position without public announcement, began a media campaign depicting himself as an equal partner in the construction of the team.
Once Jimmy realized Jerry wanted to make football related decisions, he knew it was over.