As a single node, you are incentivized to grow because it generates a positive spiral (other people open to you since you are well connected, which in turns makes you more attractive to prospective nodes, yadayada) and then you can charge more fees because of your privileged position.

But eventually, you will get too greedy, which will incentivize prospective nodes to turn to smaller but cheaper nodes. The old Bezos': "your margin is my opportunity".

And so the topology will keep on changing as these and many other factors balance each other out to equilibrium.