All customer specific Model Manger environments have been extended by years of customizing and additional applications. We are talking about complex ERP integrations, interfaces to purchasing and sales portals, ECAD and CAM integrations, automated manufacturing data preparation and optimized access via internet technology. All these business critical processes have to work again in a similar way after the migration to Windchill.
An additional complexity by migration in 11.2 and subsequent migration to 12.0 or future versions is no longer manageable.
MME and Bulk Migrator support for MM 20.3 and Windchill 12.02.0+ is a must for future migration projects.