For starters that would revive the 3rd party ad server ecosystem, since currently the only way to access full AdX demand is to run DFP. It would put AdX on the same footing as others, meaning buying through Google wouldn't give buyers a price advantage. It would allow for privacy-centric ad networks on vetted, premium publishers to emerge. Maybe even header bidding could die, meaning front-end performance would increase 10X on many publisher sites (many header bidding stacks make 300-800 ajax calls in the first few seconds of page load!) which would eliminate the need for AMP. It would mean Google would have less influence over which publishers get to monetize their content. I could go on...
