That is a good point, but considering the fact that it was inspired by a lack of resistance to Japanese encroachment by Chiang, I would imagine the warlords would be even more pissed by the forties, when the Japanese had taken significantly more territory in China. Then again, maybe I'm wrong, since I've never come across any indication of what warlord opinion was like in China at the time.
You know what this thread needs? Tailless_kangaru.
Assume I read all threads in World History.

If I don't post, it means I feel I don't know enough about the subject, or I feel I don't need to add anything to the discussion.
Anyway... the thing about the warlords is that they were a rather nebulous group. There were a few who collaborated with the Japanese, openly or otherwise. But the vast majority of the KMT-aligned warlords remained loyal, even if they don't always agree with Chiang Kai-shek's leadership.
Negotiation or defection to the Japanese would be extremely dishonourable and a truly desparate move, and the situation in China wasn't bad enough in 1940 or thereabouts to trigger it. There was stalemate on the frontlines. The Japanese had control of the major eastern cities, but they didn't control the countryside. Even so, their resources were already spread thin and they had to rely on Chinese collaborationist armies to keep the peace. The Chinese were willing to wait for the Japanese to weaken from exhaustion.
The Japanese could have certainly try to buy the warlords off, but then the question becomes whether the Japanese would be willing to do so, and how much Japanese control would the warlords themselves have tolerated (if the details of Yan Xishan's secret negotiations with the local Japanese commanders on Wikipedia is to be believed, it seems that major warlords like him would not have easily consented to being a mere puppet of the Japanese).
A major Japanese victory would change this picture significantly. Ichigo in 1944 was quite disastrous for the KMT and if China had been fighting Japan alone it's likely that they would seriously consider suing for peace, especially if the Japanese capitalise on their victory and threaten Sichuan. Though as others have been saying, whether the Japanese succeeds in holding on to its conquests depend on their ability to suppress the inevitable insurgencies and then either craft a credible national Chinese government, negotiate with individual warlords, or using brute force to impose a colonial regime. The first two would from the Japanese perspective compromise their authority, and the latter would seem impractical in the long term.