Should self-hosted deployments be vendor or customer managed?

For example, many cruise ship companies really just specialize in selling cruise tickets and operating cruise liners. All of the advanced equipment and engineering is handled by separate entities who manage the OT, and in some cases, have contracts forbidding the customer from having any sort of privileged access to it.

Similar things occur in the US military, where branches must order a Boeing / Lockheed Martin technician to do anything beyond basic maintenance of certain systems.

I believe it was (and still is?) the case with IBM mainframes?