Decommissioning is legal requirement for certain technologies. Other times it is done to prevent reuse, for liability or bad optics.
I have a 1974 Pattern M151A2 1/4ton Jeep that was cut into four pieces before it was sold as scrap metal. This vehicle was unsafe on sharp corners, and the CF grounded the whole fleet a few years after buying it, just to install a massive six post rollover cage. The army thought they'd solved the problem, but smart guys know how to weld and spare parts are widely traded. There are at least six roadworthy M151A2s in my larger metropolitan area.
MLVW trucks have been sold, but no provincial licence bureau is allowed to issue plates for road use. I hear tell, the CF was embarrassed that one would lose its brakes or a wheel and kill someone - while a sizeable percentage of the MLVW fleet is still in service. Err, that is while part of the fleet is still in use and hasn't been cannibalized for lack of spares.
Almost anything armour is covered by international arms agreements. Supposedly to prevent arms proliferation. However, odd stories come up periodically. When Gen Hillier announced Canada was getting out of the tank business, some very well heeled and well connected collectors bought and exported a few Leopard C1 tanks to Europe. Then Afghanistan changed his mind, and the For Sale sign was removed. Two decades ago when 4CMBG was collapsed, a Dutch dealer bought up a block of PCC'd Lynx recce vehicles. A few years later, photos surfaced of Iranian troops lounging around on very Canadian pattern Lynxes. To some these were cute little recce call signs; to others they were sunk money on scrap aluminum and hardware.