I'm not sure how well "short" and "decentralized" go together. The problem is that there's no good way to prevent someone from creating as many short identifier

I'm not sure how well "short" and "decentralized" go together. The problem is that there's no good way to prevent someone from creating as many short identifiers as possible and taking them up forever. Distributed, maybe, if you have a central issuing authority who publishes a complete list that others can mirror and archive. I think this is what a DOI is supposed to be. IPFS.io itself is an IPFS gateway, but you can resolve /ipfs/ paths without it if you run your own IPFS daemon; in fact this seems to be the end goal of the IPFS foundation, with ipfs.io being an stopgap measure since most people aren't running an IPFS daemon yet. The easiest way is to get this is to install the IPFS Companion browser plugin, which takes over resolution of ipfs.io and replaces it with a local gateway on your computer.