To expand on my incredulity , first, there are a plethora of FB equipped single engine boats in all lengths and styles, especially "trawler style". The very popular Mainship 340/350/390 being one of them for instance, including the one I took up the Napa river. Nordhavns, Monks, old GBs, Cape Dory, and so on and so on.
Second, I guess one has to pick their paranoias. Certainly, for river, slough and ICW cruising, I would not give up a FB for whatever running gear protection a single allowed. If I could get a single that had a FB and the layout we wanted, sure, that'd be great. But to give up so much cruising pleasure for a little extra protection for a not-that-common incident, not for me. I think of the hundreds of miles cruising the Delta in a Mainship 430, including one winter when the rivers were really flowing, or the thousands of miles on the ICW (most of it like the Delta on steroids), Hudson, Connecticut, St Johns rivers in our Hatteras... just no way without an FB.... and no major running gear issues, and one minor one. . Even with our 30x29 4 bladed props, it only cost about $250 to fix a minor ding and refurb and computer balance the entire prop. I should note the Hatt has a deep keel the depth of the props. You can ding the prop on flotsam on a single too.