Take you standard Amazon warehouse for small articles as an example. Maximizing untilisation, meaning the ratio between space used und spa e available, in that case you end up with a never ending mixture of differwnt small articles in the same location. Without RFID chips on every article, which is as of now prohibitively expensive, the only feasible way to count the inventory is using people.

For other warehouses and articles, like white goods or other large stuff that easily identified with out touching it whether a drone, a ground vehicle or something running on rails is better suited depends on a couple of factors. If you might run into situations that require a chenged warehouse layout, which occurs much more often than you think, fixed installations add a lot to the fix costs and rwduce flexibility.

Using ground based solutions runs into issues with high warehouses, storing up to 15 meters in height is not uncommon. Again, density is the main reason for this, getting more inventory at the se number of square meters. In that case a drone is both more flexible and can cover the heights as well.

But thats theory, not sure what came out of the Fraunhofer trials covering white goods.