The young and aggresive ESRI's compaetitor, TerraGo, recently introduced the Over-Zoom feature that allows the online web maps to zoom in indefinitely and still dispaly the "canned" basemap layers, however low resolution they may have. Just like you always were able to do in ArcMap. It's still not the case in the ArcGIS Online application. Esri's better to catch up with what other people do.
This idea has been merged with Zoom beyond basemap in AGOL.