Switching costs and risk are the major reasons these systems stay around forever. If you've got something that works. Even if it's sub-optimal, you're never going to move away from it. I worked at a place that used an ERP from a company that went out of business, so they would install DOS emulators for a really ancient version of DOS on every computer for people who needed access to the ERP. This was seen as a better solution than the perceived risk of switching ERPs. This was 10+ years ago, but to my knowledge they're still doing this.

Apparently Stanford had its own homebrew system, and some dean had decreed that they had to move to Oracle Financials. For literally years, these poor people struggled with it.

I never worked with that or SAP so I can't compare them, but I think both are the Full Employment Act for consultants.