It’s a reasonable fear, definitely something to consider. It’s also true that If that’s the fear, most of the battery-operated stuff that works with smartthings won’t help. Even your minimotes aren’t going to work if smartthings has flaked out. There have been at least three minimote failures in the last year for people using the stock device handlers:
http://thingsthataresmart.wiki/index.php?title=Bug:_First_Reports
For that matter, the iDevices instant switch could also fail since it doesn’t provide a mechanical relay method of controlling the lights either.
The best way to address your wife’s concern is not to use any battery operated devices at all, but stick with switches that will operate as conventional relay switches if the home automation system has failed.
Or at least make sure there is a conventional relay available for every light, even if it’s not in the most convenient place, and use the battery operated devices as a secondary means of control. Then if you do that, there’s no problem with using custom device type handlers, so you get more choices again. 