Users could use SPV clients in case of resource constraints, on the other hand there are incentives in place, that till now, have kept miners from shutting down their nodes.

Bitcoin core latest version (0.10.0) have reduced significantly the bootstrap time of a node (e.g. through "header-first" mechanism).

Future releases will introduce other improvements that will lead to a reduction in terms of network bandwidth/latency (O(1) block propagation, IBLT) and IO space (e.g. pruning)

That said it's true there's no economic incentives in place to "just" run a full node. Person that run full nodes does that for a lot of reasons modulo the economic one.

Hence the number of full nodes is steadily declining http://www.bitcoinpulse.com/#/chart/bitnodes/num_nodes

There are proposals (1) floating around to fix the situation through, guess what... economic incentives, but we are for from getting some sort of agreement, worse than that I dare say that in this case we're still struggling with problem awareness.

(1) https://bitcoinism.liberty.me/2015/02/09/economic-fallacies-...