How it could disincentives filling the last spots is that the total cost of the static plus the fee doesn't make sense because the returns on lets say a L3 node would be to long term. I would like it not to be a predefined fee, but one that responds to the market demand for the nodes. If nodes are not being filled after a period of time the fee would be lowered. If all nodes are filled maybe the fee increases to discourage people from breaking their nodes on a whim of a price increase.