Nothing. Linus doesn’t personally do coding on the kernel, he has a team who do that and he oversees it and makes the hard decisions.
There are others who will take his place and the work will continue.
If somehow the entire kernel team shut down, Google, Samsung or some other large corporation would take it over and continue development because at this point many, many, many servers, phones, smart devices, iot, and other appliances rely on the Linux kernel to function.