A complex ban does solve the problem. There is nothing uncompetitive about using Sand Veil Garchomp without sand, and your opponent having Tyranitar or Hippowdon to activate it. This is just simple risk/reward like Surf vs. Hydro Pump. Under a complex ban, they will be literally the exact same situation. You can't get the benefit of rain without the risk that your opponent will use 5 Swift Swimmers. This can be the same for sand too. The bullshit uncompetitive part about Sand Veil is that you have absolutely no control over whether your 100% accurate move hits or not. However, under a complex ban, if you miss against Garchomp with a 100% accurate move, it could've been avoided if you just didn't bring your own sand. This isn't even including the possibility that people will choose the effects of Rough Skin, such as combating VoltTurn, over a Sand Veil that they can't even experience the effects of in most of their matches since they can't set up sand themselves. So yes, the main reason for a Sand Stream + Sand Veil ban would be to preserve the viability of Pokemon such as Cacturne. However, you can't really argue that Sand Veil is 100% bullshit even under a complex ban, because if you've read the rest of my post you'll know that it's not.