That's actually a good point, but we know that ChipMixer's reserve pool isn't all that large. Imagine how that will be in a situation where people see this as an interesting money making oppostunity. ChipMixer will get 'spammed' with mixing tasks, which eventually would result in a lot people desperately complaining about them being handed a voucher, while they had their eyes on receiving private keys with BG connected to them. Then new chips have to be prepared, and these new chips will not come with BG anymore. It's basically a first come first served situation. I personally will not do so just to obtain BG, because I find that people needing privacy shouldn't suffer any delays due to greedy people emptying the reserve pool.