The logical conclusion of everyone taking the model of "pay me to review patches or read your issues" is fragmentation. Instead of a vibrant ecosystem where people collaborate on patches and the project grows, the issues and solutions remain internalized in private forks. Is that a better outcome? From the time/money standpoint it's great, and it may make the project more sustainable since there are fewer issues and PRs to review, but it hurts the project long term.
You have articulated well the trade-off between "project first" and "business first".
From a user point of view, project first is the only thing that matters. There is an instinctive resistance to anything that gets in the way. On the other hand users generally don't care about the project authors getting paid or not.
From the author side, getting paid is often a goal. If that means limiting the community to paying users, then so be it. From their perspective they are offering value, and they choose to spend their time focused on users who are paying.
In other words they need your money, not your code suggestions. They can code, but code doesn't pay rent.
As you say, others can choose to silo their changes, and not post them back, that is their right, and indeed likely happens anyway.
