Scrum should at best be viewed as training wheels for agile teams. Not the end state. Good teams abandon scrum for Kanban / Lean concepts once they start building good collaboration structures, better communication with business, earn trust by delivering and recognize (as you point out nicely) that the sprint timebox is artificial B.S. in practice. I think the Parkinson's law can be a common trap for teams to fall into with the Sprint timebox malarkey.