SoftBank is famously opportunistic, and was probably going to be the first stakeholder to walk away regardless of how successful AI was. They're always looking to stimulate synergy with their investment portfolio, and it's probably dawning on them that ARM needs Nvidia more than Nvidia needs ARM. Similarly, none of Apple's AI hardware IP is feeding back upstream to the ARM standards, so investing in OpenAI for them is basically paying their competitors to clobber them. Pending a serious sea change, it never made any sense.
I'm not optimistic for OpenAI, but SoftBank has made this mistake before, and was pretty apparently doomed from the start.