We have multiple scenarios where we would like to have cached layers above feature layers in AGO maps. Some examples would be:
1) Cached complex highway shields
2) Cached sign images as symbols
Reference from AGO documentation stating that it can't be done:
Layers are always displayed on top of a basemap, and feature layers are always displayed on top of tileset layers (map, image, and tile layers). Depending on the type of layers you have in your map, you may not be able to move them all up or down. For example, if you have one map layer and three feature layers, you can reorder the feature layers, but you cannot move the map layer on top of them. You cannot reorder a KML layer or a basemap.