To be clear there is little risk in anyone manufacturing truly proprietary chips. ARM licensing ranges from "you shall produce exactly to spec" for those wishing to vertically integrate commodity parts to "you may freely modify" -- but in either case it's the ARM design being licensed. It does nothing but harm the licensee to build something novel and strange functionality.
This system has been working quite well for Arm for decades at this point. If anything, the fact that well-known brands like Apple and Google are spinning ARM chips speaks volumes to Arm's excellent business model. Nothing we've seen from either tech giants is truly revolutionary -- the smaller fabs have been doing the same things for years.
Why anyone ever sold Arm, I will never understand.