With respect to the missions the US government cares the most about, the US did have replacements for the Shuttles. Namely Atlas V and Delta IV. After ISS con

With respect to the missions the US government cares the most about, the US did have replacements for the Shuttles. Namely Atlas V and Delta IV. After ISS construction was finished, there was little point in keeping the shuttle around just to ferry people around unsafely. Launching people into space is more of a side gig to keep a steady stream of young idealistic recruits coming in. Letting that lapse for a few years was demoralizing (less demoralizing than losing a third shuttle would have been) but not a real problem for the US government otherwise. Incidentally, Atlas V used/uses Russian engines. Pretty bad idea in retrospect but at the time a lot of people thought it seemed reasonable.