I still don't really see the issue here.
The donor gives up $1m, suppose he has a marginal tax rate of 40% on his income, he can subtract the $1m from his taxable income and save $400k in taxes at the cost of $1m.
The donator is economically worse off. Even in the best case scenario that a $1m donation reduces his to-be-paid taxes by $1m (is that how it works in the US?), there's still no gain. Instead the stock could've been sold for $1m, and there'd be a hypothetical future $1m tax payment, so he'd be left with $0. Or he'd give away the $1m and there'd be hypothetical future tax savings of $1m, so he'd be left with $0, too.
As for the DAF, he can never retrieve that money, nor spend it on anything that's not for the public benefit. Donator has no economic benefits to gain. Perhaps prestige, or donating to a politically-coloured charity (e.g. a charity that informs on abortion rights), but there's no direct economic gains to be made here for the donor right?
I can see why it's problematic if donated stocks in a DAF depreciate. But I see no reason to believe that would consistently happen, nor why there's any reason to not liquidate the donated asset right away and invest the cash into a diversified portfolio until the money is spent on a charitable cause. Typically stock-markets rise in the long-term, so it's more likely that appreciation will occur, in which case the 'subsidy' of a $1m tax break for a >= $1m asset seems well spent.
Would appreciate more info as I've got very little knowledge on this and I don't really see the big issue.
The only issue I see is if assets consistently get misappraised, e.g. a $1m asset appraised at $10m, generating a huge tax break. But that's fraud, illegal.