> in a single-party government, how much will it really matter?

There are still intra-party factions. Xi’s anti-corruption drive is an effort to defang them. An arguable strength of China’s one-party system was that it had an odd sort of internal democracy, neatly contained to the party elite. Xi is disabling that internal disagreement, which bodes well for short-term stability but points, long-term, towards China becoming a normal sort of dictatorship.