Shutdowns
Multiple suppliers for key components us a terrible idea. Tremec isn't exactly about to hand over it's DCT design for some other company to take away their contract numbers. And using two different DCTs would go over so we'll in a high performance vehicle.
Not to mention the gap in quality and performance in either case. A perfect example is quite possibly the phone in your hand. Apple uses both LG and Samsung for it's screens, and one of those two has major blue shift and image burn-in in the greys. If you have a Samsung phone then you probably already know how much the rest of the world wishes they got the US models.
Things like the DCT and it's parts, radiators of a specific size and shape, and a thousand other parts aren't quite as interchangeable as tires. There's no industry standard DCT. I'm sure there are dozens of manufacturers who would be happy to set up a production line, and hire and train employees, just in case GM decides to call them.

Given that the DCT is a Tremec patented unit, it's not like GM could just go to another transmission supplier and say "Hey, we want you to build half of our transmissions in case Tremec can't meet the demand. And for parts where GM could have multiple suppliers, none of those suppliers are going to be geared up to be able to double production in an instant. No company will build more capacity, or hire more labor, than what they need to meet demand.
To be prepared for supply interruptions, GM would have to order more parts than they need and stockpile them in or near Bowling Green. And maybe GM (and pretty much every other manufacturer in the world) should have been doing that. But doing so increases costs, and your Corvette would cost more. JIT reduces cost, which is why the entire world adopted it.















