And what is the revenue stream tied to that ci/cd pipeline they aren’t capturing today? Apple would sell less hardware in order to…?

There aren’t any app developers avoiding the Apple ecosystem because there aren’t Darwin containers. They don’t sell server hardware and by all accounts have no intention of ever reentering that space. So they’d spend a bunch of developer cycles to reduce their own revenue stream with no apparent upside beyond “goodwill” which they’ve never been overly concerned about.

They already support this scenario with XCode Cloud, it is only a market for those that don't want to pay Apple for it.