> This centralization is by design: the winner of presidential primary effectively becomes "head of party" and installs their own leadership at the national committee level.

That's not really true; the winner of the general election becomes effective head of their party; a primary winner who loses the general generally has less (but significant) influence. Hillary Clinton has less control of the DNC after the 2016 election than before it (she had unusual influence before the election for a number of reasons); the reform eliminating first-ballot superdelegate voting rights was due to influence gained by the Sanders-aligned faction.