A damn good question. These old school mega merges are widely avoided in most places these days, and with good reason. So the skills for doing this are thin on the ground. Plus it’s one of the largest projects in the world and whilst Linus got to grow into the current role over decades, a replacement would be starting on an already speeding train.

An obvious solution is modularization and stable internal ABIs, but the Linux community have avoided that approach for a long time, and with good reason.

Whatever happens I hope (not for me/us really, but for our kids and next generations) there is a plan and will materialize as single upstream.

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.

GKH takes over?

IIRC he's already the maintainer for the "stable" branch so a lot of the work people think Linus does is already on his plate.