Epic/Cerner/SAP may be CRUD apps with clunky and outdated interfaces, but I wouldn't classify them as basic. Those applications are massive and have a ton of legacy code to connect different things. I'd suspect much of the rest of their staying power does lie in the company's scale and relationships, as well as their domain knowledge. It would cost massive amounts to build a viable competitor to any of them, and you'd need people who understood the problem space well enough to know what code to write. Doesn't matter if it's just a basic CRUD app built on top of a standard RDBMS, if you don't know what features are needed.