Do you think IBM wouldn’t make Red Hat completely proprietary if they had the chance?
Adding to this, Google would make Android fully proprietary in a heartbeat if they could, given they're already closing down more and more portions of the AOSP and trying to lock down app development and distribution as well.
And conceivably all it would take to turn Android fully proprietary ala Windows, is to hard-fork AOSP to keep the Lineage/Graphene/etc. users happy, and then rewrite main Android as closed-source.
Although, it's kinda ironic that Windows, a fully closed environment, is less restrictive in terms of app dev and distribution, than Android, a supposedly semi-open environment, is. Like, MS isn't mandating signed exes or trying to fully lock Windows into the MS Store, yet, while Google is trying to mandate signed APKs and also trying to lock Android into the Play Store.
And before anyone says, 'But SmartScreen,' unless that option is specifically disabled, you can just run unsigned exes by clicking 'Run anyway' still, Android doesn't have a 'Run anyway' equivalent option AFAIK.