You can have pilots flying jets around at 1500 hours safely, provided they are trained extremely well and to great detail, and face a stringent selection and testing process. This is how it is done in the military and at some foreign carriers.
Sure, they may only have a few hundred hours, but they're the best of the best and trained by the best. That is not what happens in the regional airline industry. Anyone with $100k and some basic aptitude can currently become an airline pilot. Then they're provided training that meets the minimum required and is, sometimes, extremely low quality... and put in the right seat of a jet with a captain who has the same "upbringing" as a pilot.
These two situations are nothing like each other and it is downright laughable when someone says "The military and airlines in Europe have 300 hour pilots!" while trying to imply there is any sort of comparison.
Unless we go to the selection and training process found elsewhere (which would probably weed out a lot more people than the 1500 hour requirement will).... we need something to weed out those who aren't up to par. The 1500 hour requirement is a good step in that direction. It substitutes high quality training and selection with real world experience. A different way to hopefully obtain the same goal of having quality aviators flying 121.