Could you imagine Boeing/lockheed/GM/some some oil company or any other manufacturer going up to their stockholders and saying: "This next year, profits will be significantly reduced as we replace every piece of equipment we own, replace every 1/4-20 screw with an M5, and redesign every aircraft/automobile/widget to meet new metric specifications"
Designing new products in metric wouldn't require old products to be replaced with imperial. Each product line has it's own special repair kit, spare parts, already. It's not like the suppliers who actually make parts aren't equipped to do metric if they're selling parts worldwide.
The UK went metric in the 1960s and it managed just fine, so there's no reason why it can't be done. Some pragmatism can be involved, for example road signs in the UK are still in miles, and one can still buy a pint of milk or beer (though the latter are technically labelled and sold in millilitres).
A move to metric wouldn't dictate the internal practices of a company like Lockheed, they could still require their suppliers to build parts in inches (even if that means that formally they purchase 2.6234cm sized screws or similar) and they can continue to do everything in inches. Not that they'd want to.