Why a submarine-battleship hybrid defeats itself

What useful niche could a 50,000-ton submarine-battleship hybrid fill?

None convincingly, because a submarine is designed to submerge and avoid fire while a battleship is designed to remain afloat under fire, producing incompatible structures and operating concepts.

The hybrid would sacrifice surface speed while imposing an enormous pressure hull, difficult stability calculations, and guns exposed to repeated submergence. Its defining features work against one another rather than creating a useful new mission.