Con: You are not guaranteed anything with a new head coach
There is a belief among fans that once you fire your head coach, you are bound to have more success. That is simply not true and there are plenty of examples of that.
Don't you think the Panthers thought they would be better with Frank Reich? Don't you think the Jaguars thought they made an excellent hire with Urban Meyer? Those are just a couple of the disasters we have seen recently.
Arthur Smith has at least shown he can keep this team from giving up. While the division has been very favorable, the team has weathered a few storms and kept themselves in the race. A new head coach would could be better or he could be much worse—you just never know for sure.
The counterargument is that you would rather have a coach who could be better (or worse) than average than an only average coach. That is certainly a valid statement.