Benitez has been told the club do share his ambitions and will back him, having come back from the brink of “catastrophe, a blood-bath" and potentially years in the non-Premier League wilderness.
Charnley said: “It was a one season gamble that was not sustainable.
“What could have happened if we hadn't gone up? It would have been a catastrophe. A blood-bath. You cannot operate a business continually that has a wage bill that is higher than it's turnover. It is not sustainable.
“We'd have had to reset, recruit Championship players, and it would have been years in order to get out of the Championship."
Charnley insists they are now in a healthy position to push on, and has insisted all the cash available gets spent on strengthening.
He said: “At the beginning of the season did we intend to finish tenth? I think no is the answer. This year was about staying in the Premier League and it's about growing and improving year on year but doing that within our means.
“That's not to say there is a lack of ambition, because there is definitely not a lack of ambition, it's just it will take some time."