It is something that could come back. One could argue that with the exception of the Marine Corps --it being part of the Department of the Navy-- the Joint Chiefs could possibly hold that rank.
During WWII, there were 13 million men and women in uniform. Believe it or not, with the Army under 1 million (counting National Guard and Reserves), and the other services cut back in a like manner, EVEN WITH COMPUTERIZED DATA BASES, there are more flag officers than we had in WWII.
I saw reports that with 2,400 men in Somolia during operation Restore Hope, there were 12 flag officers in theater. 2,400 men is an understrength command for a full Colonel. Makes me wonder what they had Captains and Lieutenants doing, Corporal of the guard work?
Back to the OP though. Due to the nature of the position, a Five Star Flag Officer never retires. The only exception was Eisenhower. Even then, his rank was restored by Congress after his Presidency. MacArthur was re-activated by Kennedy in 1962 to mediate a dispute. That is a lot of money to pay for a men too old to have active commands.
Really, what was somebody like Omar Bradley going to do in his 80's? The man was 88 years old, and still drawing a full Five Star General's pay when he died back in 1981.
Really, right now most army commands exist only on paper. With 10 Active Divisions, it isn't a command fit for a Five Star General.