I think for one warlord to survive without overt foreign backing, you need many warlord powers to survive if they are to withstand the forces of centralization.
- Japan takes a slightly less overtly militaristic stance in China starting in the 1920s, focusing more on trade and business.
- Chiang Kai-shek is sidelined when the continued existence of the Fengtian clique under Zhang Zuolin means that TTL's equivalent of the Central Plains War does not result in a clear victory for Nanjing.
- KMT Central has to share more power with warlords, especially those in the north. To check Zhang, Chiang has to allow folks like Li Zongren and other southern warlords a higher degree of autonomy.
- Sino-Soviet border war of early 1930s, plus expansion of communist guerrillas to the northwest and northeast moves Fengtian clique and other northern warlords into the Japanese camp
- Japanese run complex "gunboat diplomacy" in China, protecting business interests and helping warlords suppress radical Chinese nationalism.
- Fearing Soviet influence via CCP in China, Japanese team up with KMT and lesser warlords to push a conservative form of authoritarian patriotism, creating a significant ideological divide between affluent urbanites and poor peasantry. Meanwhile, economic development from 1930 to 1950 increases the bourgeois class.
- From 1950 to 1970, anger at the incompetent and corrupt Nanjing govt is leveraged by some local leaders with warlord connections to push for greater federalization of China. This is especially true in the south, where the inability of the central government to enforce Mandarin education means greater space for expression of local dialects.
- The gradual decline of Japan as a colonial power after 1950 forces the KMT to accede to more domestic and regional pressure.
- Around 1980, de facto federalization is achieved courtesy of massive demonstrations throughout Chinese cities, at the same time ushering in speedier democratic reforms.
- The central government, having struggled throughout the 20th century to unify the military and other state institutions, moves closer toward this goal, but finds it convenient to allow many aspects of the warlord system to remain, especially in the undeveloped countryside.
- By circa 2020, China is a mostly modern and developed country -- in the cities and coastal provinces, at least -- while "warlord" clans run things in the provinces. Most of their influence has since been merged with official institutions and corporate interests, but they retain, for instance, local militias and police. Much of contemporary politics in TTL China in 2020 centers on the struggle between "centralists" who want to bring better rule of law and social equality to these backwards regions (think something like US "flyover country," but with around 2 billion people in China) vs "regionalists" who are all about preserving regional political and cultural autonomy.
- "Centralists" are characterized by cosmopolitanism, focus on western-style legal reform, and using the power of the state to enforce civic nationalism. They often are either secular, believe in Christianity, or follow some modern version of Buddhism/Confucianism. They look up to the example of India, a country that following the end of British rule successfully ended most ethnic strife with significant legal and social reforms.
- "Regionalists" practice Daoism and folk religions that classy Chinese urbanities look down upon as barbaric feudalist superstition. The idea of "four or five generations under one roof" is seen as the ideal of social existence, polygamy remains common, and some places have not even given up footbinding. They are intensely skeptical of government efforts to implement reforms, such as forcing them to follow laws of commerce and educate the youth in Mandarin.
tl;dr: One Country, Many Systems.