So the situation would be: you create a setup, buy devices and then they randomly shutdown as they pull too much current? How is that better than having a well defined standard that ensures compatibility?
The device first negotiates a certain current capability. If a device explicitly asks for 15W and then goes on to draw 60W, you can hardly call it a "random" shutdown: it is clearly misbehaving, so it is best to shut it down to prevent further damage.