ESRI and the USGS provide WMS tilesets of USGS topographic quads. So a generic WMS viewer capability with the ability to save cached tiles offline in Gaia could provide that functionality without incurring additional bandwidth costs. Except of course, users might prefer these maps to the in-house ones that you charge a subscription for.

The subscription model has distorted the smartphone map app ecosystem, just like it has the rest of the software industry.

For the record, OSM maps lack the detail and reliability of the legacy USGS 7.5 minute topographic quads. For that matter, the most recent USGS topographic maps are themselves almost as bad as OSM! For best coverage I would recommend the 1990s series or earlier. ( https://www.usgs.gov/faqs/how-do-us-topo-maps-differ-histori... )

For Android, OSMAnd is good enough for navigation and Qfield can load any GIS vector or raster layer from a desktop QGIS project.