> Another issue is that the tax savings felt by the donors is tied to the original contribution's valuation and not the valuation at the time the money is used. So in the case of GoPro's Nicholas Woodman, he got a tax break based on $500 mil, but the shares have dropped around 93% so if they're used for charity now it's like he only donated a fraction of that amount.

Sure, but the reverse is true as well - the contribution could increase in value, which benefits the charity. And if that happens, he doesn't get to claim the difference as a deduction.

Furthermore, he gets the tax deduction based on $500 million because that's the amount he's giving up at the moment he makes the (irrevocable) decision to divert the funds to charity. (The alternative is for him to liquidate and hold onto the $500 million himself; he's getting a tax deduction for not doing that).