Your question is traditionally phrased as "what if Linus gets hit by a bus?", and if you search the web for variants of that phrase, you'll see it being discussed as early as last century (it didn't take me long, for instance, to find a slashdot comment from 1999 mentioning that hypothetical scenario). The answer back then was "Alan Cox takes over"; the specific maintainer who takes over has changed over the years (nowadays it's probably Greg Kroah-Hartman), but other than that, the answer has remained the same.