I think suggesting that Linus "needs to" do anything is missing the whole point of "freedom".
Linus doesn't need to do anything, and the community doesn't need to pay any attention to what he does. But as long as what he does is useful the community will pay attention (a statement which applies equally to lots of other people).
If Linus chose to appoint a successor that might be useful ... as long as they aren't on the same bus that takes Linus away from us... But it would be no guarantee of success.
Were Linus to disappear other developers would be free to try to create their own 'central' tree. or not. Maybe several people would try. Ultimately one "winner" would emerge largely because one central tree is more useful than two (and maintaining that central tree is probably a fairly thankless job).
You might see the time with 2 (or 3 or 4) 'central' trees as wasteful, but that is also quite normally in our community. A lot of code is generated and then discarded. But this isn't a waste, it is a learning process.
A bureaucracy appoints a successor, a meritocracy allows a successor to prove themselves.