It's not a bad hypothesis at all, there is solid basis to proposing that it exists, its just that how would you go about confirming it?

The logic is that comets are inherently not a stable phenomena, a single comet can't keep going for geological time periods. So somehow you need to have a mechanism for new comets to appear, there needs to be some sort of reservoir of potential comets yet to become comets. If you do a bit of statistics based on that idea you end up with the model of Oort cloud that we have now.