The seating configurations for United and Alaska, the 2 major customers for this aircraft, don't require this plug-door feature. Structurally, for these large carriers, the plug door serves only as a fuselage weakness and failure point.
Why don't they just rivet it permanently to the fuselage and make it non-functional? Per the diagrams shown, it's a complex assembly that serves no purpose at all for these carriers.
(Apparently it was used as a cargo door to furnish the interiors, which seems like a trivial use case.)