I've argued that heirs, if they don't have proof of the basis, should have to treat the basis as 0.
Why should the basis be what it is today vs zero? It's pretty simple, if person dies on day N, and we know what we would tax the item as if he sold it on day N-1, then we should be able to know what we should be able to tax it at if sold by the heirs on day N+1.
Zero would acknowledge that the heirs paid literally zero for it. conversely, if the the asset tremendously rose in value (i.e. an order of magnitude or more). then even with knowing the cost basis, one can effectively treat the cost basis as zero without significant effect on the heir (i.e. at a 33% tax rate, if it grows by an order of magnitude and you treat it as 0, instead of 10% of the current value, you only increased the amount of tax paid by 3.3% of the value). If it grows by even more than an order of magnitude, than the percentage drops (2 orders of magnitude and you are at 1/3 of a percent extra vs treating it as zero).
i.e. there really isn't much of a loss to the heir if one has to treat the asset as zero cost basis. and one can always provide said cost basis in the cases where the asset growth can't be measured in even a single digit order of magnitude (but once one breaks a single digit, then there's very little loss to the heir).
In general, I dislike trying to treat different assets classes differently, the rule should be no step up, track cost basis or you as the heir have to treat it as zero.