That would create suely create issues with defending teams deliberately collapsing 5m scrums, making the laws look like an ass. To go from free kick, free, kick, free kick all over the pitch to yellow card or pen try for 5m scrums as we currently do would seem inconsistent to me, but if you don't allow repeated 5m scrums following defensive infringement you then get the issue that the defending team will deliberately collapse any 5m scrum knowing there will be just one, and whilst repeated reset scrums are a pain, there is huge drama in repeated attacking 5m scrums where the attacking team opts for a series of scrums. That is the problem with a scrum, it is both a huge pain that interrupts the cadence of the game and a source of great excitement and drama, especially the 5m scrum after being held up.