The premise is questionable as said above.
First, simply killing Chiang would not result in "KMT staying left wing" and prevent the KMT-CCP split. The cooperation was already uneasy due to the CCP's rapidly rising influence within KMT rank, the secrecy in CP operation and their adherence to Comintern(and in KMT's view, Russian)leadership. Many KMT old guards were fearing that they will soon be sidelined in their own party by an alien influence. Without Chinag, this right wing old guard in KMT is still powerful. Even the left wing elements within KMT were unhappy with the "dual leadership" of the left, and wanted to dissolve the CCP to consolidate the left wing.
In short, to get this result you need the ccp to be a very different ccp and KMT to be a different KMT.
Second, assuming the KMT-CCP united front did survive, it doesnt necessarily mean the end of warlordism in China, in fact quite far from it. Warlordism(especially the so-called New Warlordism after 1927) came from many different factors, and many wouldnt be solved by this PoD:
1)The inherently fragmentary nature of KMT military. Apart from the core of Whampoa cadres, KMT army is mainly built from a set of different provincial forces, primarily the Guangdong Army, Guangxi Army, the pro-KMT exiled Yunnan Army, etc. While loyal to KMT ideology, these forces are highly regionalistic, as soon as the KMT expand from their base Guangdong into a national regime, they will naturally split into their own cliques.
2)In the course of the Northern Expedition, a lot of victories were won politically, basically thru buying off/persuading the warlord army to defect to /ally with the KMT. These former warlord forces absorbed into KMT has less loyalty and ideological attachment to the KMT ideal and would have even less restraint from becoming new warlord - unless they are properly disbanded, which a successful KMT-CCP united front might be able to do.
3) Threat of foreign intervention and Great power sphere of influence. China has long been carved into sphere of influences by Great Powers since the end of 19 century. The regional warlord is often propped up by Great Power as a proxy in their sphere of influence. The Fengtian clique, based in Chinese Northeast(aka Manchuria) is the most prominent example. The KMT's attempt to conquer those warlord would naturally be seen by Great Power as an intrusion into their sphere, and cause much alarm, more support to said warlord, and even direct intervention. In the case of a KMT-CCP united front this would be much more alarming to Great Powers since the KMT would be viewed as borderline communist, Soviet proxy, and highly anti-imperialistic.
So assuming the global situation remains the same, what would happen in your TL is a deeply heightened Great Power resistance against the Northern Expedition. It's likely that Britain and Japan would agree on joint pressure or intervention against KMT. Dont forget that even in OTL Japan sent a division into Shandong to block the KMT army from advancing and caused a limited conflict, and ITTL there will be more. This would cause the KMT to rely even more on the Soviets, but it's hard to say how much the soviet would be able and willing to help. If KMT tries to go into Manchuria I think it would freak the Japanese out and cause more direct intervention. If the fengtian clique is deemed unable to fend off the KMT, it's likely the Manchurian Incident will be sooner(Kwantung Army was already planning on direct occupation of Manchuria in 1928). If the situation escalate into a de facto KMT-Japanese war, I have doubt that KMT could succeed, and should the Northern Expedtion be beaten back a coalition of Great Powers(with Japan at the forefront), then you are dealing with not a China without Warlord, but the very opposite.
