It's the correct decision for Ben to retire now rather than trying to prolong the misery.
Thing is that the Steelers aren't aggressive enough in the market and I doubt next year's draft class will be deep enough for them to pick someone up. The issue is that there are a lot fewer good enough QB's than there are teams in desperate need of one. I reckon Russell Wilson will be on the move in the summer, but I can't see the Steelers throwing the kitchen sink to get him even though he's got the potential to be good well into his 40s. Then if Watson is cleared to play next year, I assume Miami and Carolina will be a lot more active in trying to get him.
What does that leave then? Seemingly a massive void and quite possibly Mason Rudolph as a starter. Tomlin clearly needs to be willing to ditch a lot of future picks to get a QB good enough to properly complement the defense. Otherwise he might start looking at HC vacancies at other franchises. As good as he is, even the best coaches need a change of scenery sometimes and I feel that he'd be well-served to try something different after all these years. With all due respect to Big Ben, I feel that Tomlin has never had a truly elite QB in all of his career and I really wonder what he would be able to do at a place where he had one or if he found someone to play in yellow and black if he stays.
In the end, what I do sort of expect to happen is that Shanahan feels the need to get Trey Lance in as a starter at any cost, Garoppolo gets released and Tomlin picks him up in free agency. Basically a clone of Big Ben in the past few years then.