The absence of the M class was an industrial problem as much as a design choice. German battleship construction consumed resources and infrastructure that might otherwise have supported the intended light cruisers. A fleet programme on Plan Z's scale would have required whole shipyards to be under construction by 1939 if it was to produce major forces in the 1940s. Naval planning depends on industrial capacity and lead time as well as approved hull designs.
Notable Point · 19:43-20:09
Why Plan Z needed industrial preparation before 1939
The unbuilt M class shows that Plan Z needed shipyard expansion underway before war began; paper designs could not create industrial capacity.