Either way, you may prefer to use a less expensive third party for maintenance or even self-maintain. If you have a genuinely perpetual license with no strings attached, you could choose either of these routes. But if you have a conditional perpetual license that requires you to stay on maintenance, you may not be able to do so without violating the terms of your contract. And before you violate those terms, be sure you know how likely your vendor is to litigate.
Many ERP vendors that still “support” older legacy products will assure their customers they will never “sunset” a product. But several very prominent vendors have now set dates for a declared end of life of their older products. At that point, they will no longer support it. These end-of-support dates are quite common in terms of supporting specific versions (releases) of products, but until recently, have been relatively rare in declaring the end of life of the product itself. These vendors typically give you at least a year (usually more) notice before you are forced to take some action.
If your solution has a declared end of life, and you have a perpetual license that requires you to remain on maintenance unless you take some action to upgrade, migrate or replace it, you are stuck between a rock and a hard place.
While perpetual licenses were once the norm, they are becoming obsolete. More likely today, your license will be valid for a specific time period. This is generally referred to as a “term” license. At the end of the term, you must either renew the license or discontinue using the ERP system.