If the past is a reasonably good predictor of the future, than Linus performance has been good. I'd suggest there are more wins to this consistency and institutional knowledge then losses. If it ain't broke don't fix it.
If something happens to Linus, also not the end of the world as others point out. The distributors etc will all start pulling from a different tree. Witness the move to a new X tree when the old stagnated. Pretty seamless. Might not be as good, but long term would hopefully stabilize.
If the scalability bottleneck is the ARM junk, there is a real possibility that the ARM churn Linus complains about IS junk and not a scalability issue.
There is a HUGE amount of work going on below linus's tree as the article points out. This is a natural area for scalability, helped by good tooling (basically Git, written interestingly by linus). I do think eventually some deeper structure will happen. At some point Linus will simply not care about some random x. Stick it below a maintainer who does.
Having the light touch dictator keeps the kernel slightly sane, reducing barriers to entry for everyone. For software that'll probably be running in the basements of our future starfleets this can only be a good thing.