I understand that this is an old idea, but I ran across it so thought I'd do some housecleaning. This is possible in ArcMap and ArcGIS Pro by using the Set As Default button: https://desktop.arcgis.com/en/arcmap/latest/map/working-with-arcmap/working-with-map-scales.htm
"To make your current set of standard scales the default that will be used in all new map documents you create, click the Set As Default button on the Standard Scales tab. To reset your active data frame to use the default of standard scales, click Load > Default Scales."
https://pro.arcgis.com/en/pro-app/latest/help/mapping/navigation/map-scales-and-scale-properties.htm
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