I think it's a valid point but compared to something like mining centralization it's relatively minor. It's much easier to have thousands of full nodes all over the world running on VPSs and enthusiast machines than to actually have a healthy distribution of miners.
I know you didn't ask about miners but even having 1000 people running full nodes that enable people to use thin clients is still pretty good while I see mining as the major bottleneck that causes the most centralisation. Much more than lack of node distribution I think.
Having a million nodes would be good too though. A million people running full nodes solo mining on their cell phones would be pretty decentalised.
There are problems with both. Mining at least, has some incentive. There is currently zero incentive to run a node at all, and the number of nodes has been dropping even as usage has increased.
The best answer I've heard for why anyone should run a node at all is that businesses such as Coinbase or Bitpay will want to support the network since their business depends so heavily on it. This is still relying on questionable economics, but even if viable, still sounds pretty centralized to me.
The two ideas I've seen to fix this are:
1. Requiring that people run a node in order to mine (and ensuring that at least part of mining remains decentralized). That could possibly work, but I haven't seen anything approaching a solid proposal.
2. Some sort of proof-of-bandwidth where you are directly rewarded for running a node. This is even less well developed than 1.
