How depleted was the US Army senior staff in 1861?

The question of Lee's being offer the position of Commanding General of the United States Army by Lincoln in April 1861, raises a question of - 'Who was left?'.

Generals Samuel Cooper, Joseph E Johnson, and Albert Sidney Johnston, all senior to Lee, had all resigned to join the Confederacy. Along with the commandant of the US Army Academy at West Point, P.T.G. Beauregard.

Throw in such other lessor notables as William Hardee, Braxton Bragg, and a couple of dozens others, I am not aware of anyone other than Colonel Phillip St. George Cooke, father-in-law to Major General J.E.B. Stuart, CSA, and Winfield Scott, above the rank of Major, in the regular Army that stayed.

Even of the Majors, such as Maj. Robert Anderson of Fort Sumter fame, and Maj. David Hunter a friend of Lincoln, and Brevet Major Irwin McDowell, a friend of Secretary Chase, a lot of these were all quickly promoted to Brigadier Generals, or higher in McDowell's case.

It seems that the United State Army senior staff almost to a man resigned and joined the Confederacy. The common explaination has always been they were loyal to their home state. But was that their only reason for disavowing their entire lifes work?