A team can definitely do worse than Tomlin, but after seeing virtually identical results for years on end, one ultimately has to take the risk in order to achieve something more.
Some argue that Tomlin should be given another shot with a franchise QB before moving on from him, but that could take years, and even if they somehow did find a top 5-10 QB in the near future, I'm not sure how anyone could have confidence in him taking advantage of it considering how the last decade or so of Roethlisberger's career went.
Given the money Rooney has committed to Tomlin, we have at least three more years of this to look forward to, regardless of results.