It is possible that being a regular node will not be so easy when the bitcoin network has grown and a node needs to receive and forward thousands of transactions per second. So the only ones that will have the incentive to be full nodes will be the miners. And everybody else might be using a "light" client that does provide this functionality to the bitcoin network.
Good point! I guess some kind of compromise could be found to ensure a healthy network infrastructure. Like for example, clients which randomly accept and forward transactions according to their resource limits. I don't think it would be too hard to implement such measures.
It is a valid concern, but fortunately the scenario can be detected (by looking at the percentage of full nodes in the network) and most probably prevented before it becomes a real problem.