There is no doubt that Osuna is more valuable as a starter for all the reasons stated. But we had BP/stopper issues and we basically threw everything at it and nothing stuck. The real issue goes back to the off-season where we knew we needed help. For a variety of reasons, saving funds for the trade deadline, signing Martin, etc, AA chose not to do anything with the BP. The BP did not stabilize until Osuna emerged - and it was after several iterations at that.
Wishing Osuna was a starter/developing in minors and thinking now that we could have solved our early season BP issues without him is "having our cake and eating it too". If AA had done something in the off-season, then maybe but he chose not to