One point to bear in mind is that converted 747s don't have the option of a nose door and there are some disadvantages to converting vs buying a freighter variant [0].
A380s also have a greater volume than a 747 but not a radically higher carrying capacity. A 747-8I (passenger variant) has a cargo capacity of 76t vs 84t for an A380. Numbers from wikipedia, excluding the freighter variant of the 748 as it's not comparing like with like. This means that there could be some cases where transporting bulky low density items would be much superior on a A380 freighter but there's not a clear cut case for it in the market.
A380s also have a greater volume than a 747 but not a radically higher carrying capacity. A 747-8I (passenger variant) has a cargo capacity of 76t vs 84t for an A380. Numbers from wikipedia, excluding the freighter variant of the 748 as it's not comparing like with like. This means that there could be some cases where transporting bulky low density items would be much superior on a A380 freighter but there's not a clear cut case for it in the market.
[0] https://centreforaviation.com/analysis/airline-leader/air-ca...