The old question of "what happens when Linus/other senior kernel dev dies" mattered far more like 20+ years ago than it does now. The kernel developers are organized quite well, Linux is in general an extremely well-organized open source project these days, and there are several who could fill in. Linus' "2nd" is Greg Kroah-Hartman, who is the lead maintainer of the stable kernel branch (i.e. the one most are using). Linus is the lead maintainer of the in-development branch.

But of course we hope that Linus continues doing this for a long time. Most people never create even one world-changing technology, Linus casually created two (Linux + Git).