A small harddrive used up by every node on the blockchain. Would be hard to allocate these resources without the nodes becoming centralized.
I disagree that bitcoind nodes will become increasingly centralized.
HD/SSD:Size/Price ratios are getting better and better every year.
I just set up a 12TB RAID volume to handle bitcoind and c-lightning, and the price was quite reasonable for what the server will be doing.
I expect it to last me several years before I need to start thinking of upgrades again.