If everything runs perfect forever. they don't need any access.
That would also be a most singular and fantastic miracle.
Fixing the exalted machine is ever so much easier if it's not an opaque black box. Fixing it within an affordable amount of downtime pretty much requires access _and_ familiarity.
The OEM's promise of support may be made in good faith, but at least 25% of the machines in a typical plant are orphans.
If I had my druthers it would be a deliverable, non-negotiable. Because I'm often the guy who has to get it running.