Being naive here, wouldn't a massive help be to not focus on detection of DoS/DDoS attacks but instead to focus on validating that IP addresses come from within

Being naive here, wouldn't a massive help be to not focus on detection of DoS/DDoS attacks but instead to focus on validating that IP addresses come from within the range of addresses being served by the ISP? It strikes me that this would prevent a massive number of amplification attacks.