Roosevelt, who as Secretary of the Navy had learned something of how a military should function would have demanded as much from Drum as he did from Marshal. If Drum resisted Roosevelts intent to modernize the US Army he'd been replaced relatively quickly. To get a non modernized army you need a different President.
As for underrating Drum; I'll not judge him specifically, but the Army had been laying the foundations for its modernization back in the 1920s. Pershing, MacAurthur, Malin, & even Sommervel had as CoS done their best to educate the officer corps & develop doctrines suitable for the mid 20th Century. They tried hard not to recommend fools and arch conservatives for promotion to the General grades. All the CoS understood the sort of Army needed to execute the assorted Color Plans needed to be far more capable than that of the AEF of 1918. Drums advance from the Field grades to his three stars in 1939 reflected the previous two decades where Pershing, Mac & the others had tried to pick officers capable of forward thinking.
Note how when Marshal started his purge of the Armys officers in 1939 Drum was one of the chosen few for the critical commands. 1940-1941 until the restructuring post Dec 1941 Drum. Lear, DeWitt, & Kruger were retired. They were elevated as the four key architects of the early mobilization and rebuilding of the army as the commanders of the four numbered field armies that controlled the actual ground forces and portions of the service/support and air corps. Drum & the other three were Marshals point men for many critical tasks, including actual field tests of the Armys doctrines in development since 1919.
Marshall had a pool of over 60 general officers to choose from for this task. He picked these four to organize a 1,400,000 man modern field army out of the 200k Regular Army, 250k National Guards, 60k Reserve Officers, & 900k new recruits. AND lay the foundation for a projected four million man Army. There were a handful of others out of the 1939 pool of generals Marshal picked for the Air Corps and the support departments. Most were retired by the time Japan & Germany declared war on the US.