Yes you're right when talking about the steady dicrease in terms of full nodes. I think that introducing economic incentives for stimulate full node operators are badly needed.

I've recently read a proposal for such a scheme written by Justus Ranvier, you can find it here:

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

tl;dr Bitcoin peer-to-peer (P2P) network needs price discovery built directly into it through micropayment channels (https://bitcoinj.github.io/working-with-micropayments)