That might work, though I'm worried that with such rules blocks would become gradually larger and over time the number of full nodes would shrink dramatically as weaker computers get separated from the network. For example, dial-up nodes would get separated right away. No one would care that dial-up users can no longer run full nodes, and they would themselves mostly just say, "Oh well, I guess my setup is too slow to run a full node. Time to switch to a lightweight node." This is probably reasonable for dial-up, but I think that it might over time spread to most people. As blocks become larger, people on average PCs would have to switch to lightweight nodes, then even hobbyists, and then even small businesses.