The SRBs don't just impose higher operational complexity, but also far higher acoustic load that requires much higher structural strength and therefore weight on everything else.
They also complicate the range safety situation and make on-pad aborts after they've been lit impossible. Once they've been lit you're going wherever they're going, whether you like it or not.
Pretty much, yes. They want to keep the people making those engines employed. I think there might be some talk about eventually going to a renewed F-1 engine with a much reduced complexity and greater thrust than the original. There is a reason why the SLS is also known as the Senate Launch System.
