If you announce that you have 4 man years free for new tasks, in a team that may not even have 4 developers, after abandoning a feature that already existed and worked, you're delusional and so is the stakeholder if he believes it.

I agree on the principles nonetheless. You want clients to write a formal request for new features, then development has a backlog of requests and can prioritize them.

Clients might not like the prioritization but that's life, limited bandwidth, it's simple to show there is too much to do and not enough resources.