I would prefer that.
App could say:
“offline map inaccessible! - switching to online map”
And the only possible answer is OK
Than user should have noticed what has happend 
2 Likes
Do not be too optimistic about the future…
SD cards are becoming less and less popular with every new Android.
And if there is a map downloader in the app,
it could only work with app’s internal storage.
What you have now with manual loading is much more flexible.
1 Like