Sorry I was using transaction fee as a catchall for all the income that would be related to owning a node minus the income from new nodes.  It's hard to argue for or against since we have no information about what the income will be at all.  And you have to include the cost of running the server for the dapps into the cost benefit analysis.

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.