My recent experience with an ERP, specifically some major bolt-on modules, was that the vendor simply made the switch to a new platform that had maybe 60% of the capabities. A roadmap (which has actually been fairly accurate) showed about 3 years to get to 90%.

New customers were pushed to the new product. Existing one were encourage to do so and temporarily live without prior features (usually with temp workers doing things manually) for a deep discount. Those who had to stay with the legacy system were told to expect nothing but bug fixes and compliance-related updates (for federal programs and reporting requirements) and that if they needed something more than that, they'd either need to built their own bolt on (there was a robust, if clunky sdk) or pay contractors to do so.

It sucked, yeah, but it seemed like a reasonable way to go about such a transition that was always going to make people unhappy.