I consulted for a company once that had made several attempts to migrate from their ERP (as well as having made several acquisitions and mergers over the decade

I consulted for a company once that had made several attempts to migrate from their ERP (as well as having made several acquisitions and mergers over the decades). When I was there the state of things was 3 ERP systems that they had built, and another 2 that they had gained through mergers, all still operating and ERPing between each other. No matter how many enormous multi-year migration projects they started, they never managed to turn a single one of them off.