Given that the DNC proposed the early debate, and scheduled it for a time in the evening when Biden would be sundowning, they have to have expected and wanted the result. Presumably so their replacement for him would have enough time to solidify things before the convention so that it can be properly stage-managed. But then why haven't they replaced him yet?

I don't think it's the MIIC; I still think it has to be DNC-internal politics.