Backpack isn't "kinda" illegal, gray-area, like AirBnB or Uber. It's straight-up, completely illegal. And this isn't municipality-level policies it's up against, it's customs and border, national law.
I can't see any future where they don't get shut down.
All it is going to take is a couple smuggling stings on the travelers, and then they will have the reputation as not being safe.
I mean, who wants to spend their vacation in a foreign prison, waiting for the consulate? Or risk losing their job for smuggling a couple hundred dollars?
Maybe whatever you take along has a bunch of cocaine in it.
I'm a pretty experienced traveller and I do people all kinds of favours but I'm fairly picky about what I take across borders for others. As in: I get to inspect the item inside and out beforehand.
I can't see any future where they don't get shut down.