If the only comparison was times in World Championships/Olympic finals, then yes, the 2023 WC final is not evidence of a decline in the event. But look at overall times. At the top, Coe, Cruz, Kipketer, and Rudisha ran times well faster than what anyone is running today. It goes further back than that as 1:43 (or under) times were common in Diamond League events in the past. That isn't the case today. You could make the same case (perhaps stronger) than the 400 is weak today as well.