I could understand this kind of take if Raheem had stopped giving 100% and was making up niggling injuries to miss games, etc. And likewise if you were calling for City to extend Patrick Roberts' contract out of loyalty as he committed his youth to you.
Your solution would seemingly be for City to "own" Sterling indefinitely - a power dynamic which would certainly prove problematic in this day and age...
In a world where humans aren't treated like cars, I'm sure there is a wage that City can afford to keep Raheem if they want and there is also a price at which they can sell him so they aren't really forced to do anything.
In reality, the challenge is that COVID means there isn't a club out there who could meet City's valuation of Raheem and Raheem's value to City probably isn't at the £300k level that he thinks he could get on a free - hence why it likely makes sense for all parties for him to see out his contract and go.