WebMap: set side loaded tpk as option for non offline capable basemaps?

[MarkusSchenardi]

There is a nice option for offline apps: it allows to define a local TPK (tile package) to be used as basemap instead of the default basemap. It can be configured using the advanced options in the offline section of the WebMap Settings. 

BUT: why can I set this property only when offline is already enabled?

Especially for Basemaps which are not offline capable it would be very helpful to set exactly this property and to provide an alternative basemap as side loaded tpk on the device.