The mile record has dropped due to equipment (tracks and shoes) and pacing innovation since the very first. And I repeat this all the time but nobody listens: modern shoes are fast because of the special foam and light weight, the carbon plates have very little to do with it, experiments have cut those plates in half and running economy remains unchanged. they likely provide important structural support for the thick foam, and carbon makes sense to use for that for weight reasons, on a budget plastic is fine.