Because previous process nodes are alternatives. The world doesn't stop. It just stops getting better, and there's a lot of pain as supply chains readjust to available capacity and retarget parts.

Previous process nodes are alternatives, but it takes months to years to redesign and manufacture devices using available parts. What if the new car supply (because cars have a crazy number of processors in them) suddenly dropped to 5% of current rates? What if you weren't able to purchase a new computer (or any parts - DRAM, GPUs, motherboards) for five years? What about appliances - if your microwave breaks, and you can only buy a used one for 4x the new price, what do you do?

It is my understanding that TSMC has made arrangements to scuttle their facilities in this eventuality, denying invaders the technology.

a chip shortage means i keep my current cellphone for a few more years, and make do with older computer and new cars won't have a shitty infotainment system, or an ARM SoC in some gadget that doesn't need to be smart. not something that causes me to riot.

What if there were no new cars, TVs, or phones made for five years? That would cause rioting - and that's pretty close to what scenario we would actually have, because it would take years to design and manufacture these very complex devices without any parts at all from TSMC.