This brings up an interesting question - what does the military do with phased-out weaponry?
The U.S. military is no longer allowed to sell functioning weapons as surplus (except for some exceptions through the CMP). Now all surplus weapons are either destroyed, given to our allies as military assistance (that's where all the M-14's went, btw), or "loaned" to other units of government. Those weapons that are loaned to other units of government, such as the M-16's currently being given to local Police Departments, remain the property of the U.S. government and can't be sold by PD's that received the firearms.
know that all/most/some Garands went into CMP rotation, but it's hard to imagine a zillion Thompsons being demilled and/or dumped into the ocean... but knowing the military, I wouldn't be surprised if that's exactly what they did.
The production of Thompsons, M-1 Carbines, M 3A1 Grease Guns and 1911A1 pistols ended when WWII ended. Those weapons remained in active U.S. service for many years later and many were just plain "used up" or combat lossed while in service.
Many more of these weapons were given away as military assistance to other governments over the years. Some went to Korea, some to Vietnam, and some to other governments.
And yes, the government did destroy a large portion of those weapons as they became "obsolete" and when they no longer wanted to store them. There a few "New in the wrap" Thompsons, BAR's, Carbines, etc, still on the Army books for use in military museums at the Army Historical Center in Anniston, but that's a relatively small number of survivors. Those will never make it into civilian hands though and most will never be publicly displayed.
The reason I ask is because in, whatever, 20, 50 years, when we move on to the latest and greatest and dump the M-16... what's going to happen to 'em?
The same thing that's happening to the M-16A1's that are surplus to requirements now. Some will be given to other parts of the government on loan, some will be given away as military assistance, and the rest will be destroyed. Remember, ever since 1986, no new machine guns can be added to the NFA registry for civilian ownership. The military couldn't release these to civvies even if they wanted to.
The only way the military can release weapons to the U.S. civilian population is through the Civilian Marksmanship Program. The CMP is specifically restricted as to what types of weapons it can accept. Full-auto and select-fire weapons are out, as are handguns.