Its not about backup. Its about environmental stress. Photos are taken all over the place - yards, beaches, malls, in good weather and bad. SD cards are fragile. Its about, If you spend a day shooting and one of them doesn't make it back to the studio, how much have you lost?
Are they, though? Because I use SD cards for photography and have for quite a few years and I take them not only to yards and beaches, but mountain peaks, spelunking in mud caves out in the desert, etc, and I have never had an SD card break on me either physically or in terms of data loss.
To be fair, though, for photos only (no video) even with high-res uncompressed RAW photos I can't imagine anyone practically needing SD cards anywhere near the 512 GB size. That's a lot of photos...
YMMV, I guess, maybe I'm totally lucky, but I have yet to personally have an incident which makes me worry about the robustness of decent quality SD cards.
I mean it does cost $35, but it's pretty much exactly a magic remote backup machine (that can also charge your gear!). Or you can spend more cash and get one with an integrated hard drive and need no other devices.