Companies already spin off android into their own custom versions to maximize profit. Look at Samsung, for example, with all of their additional bloatware.
Android is open-source. Closing the source code for android would be so devastating for the platform’s app development, independent security researchers, and manufacturer customization, that it would probably hurt them more to lock it down than to keep it open.
If an alternative, entirely community-supported fork of Android were to be copied and maintained from the main branch of Android, it could still use every single APK that was available on the Play store, and every alternative app store, with no issues.
Sure, Android would likely lose some of the Google R&D money, but what has Google used a lot of that money for? AI features nobody asked for, benefits that only come from the use of Google’s entirely separate apps on the system, and system improvements that could be worked on with relatively similar speed by outside alternative ROM teams.
Plus, Android uses the Linux kernel, which is already supported by outside developers, and often gets security fixes that are pushed to Android without any involvement by Google in the development of the fixes.