Just a little clarification here, ENS isn’t a “database” per se, at least not in the way that most people consider to be a “database”. The ENS smart contracts, registry entries, and records are all stored on the Ethereum blockchain, which is a completely public and permissionless network. Typically when people refer to “databases” they mean a private (or permissioned) data storage solution that is centrally controlled by some company.
Nothing, assuming you do the legwork to get that implemented. You could come up with a record standard for that, get the ENS community on board with it, get browsers like Brave/etc to implement it, create pull requests in various projects where necessary, etc.
But if your goal is to “setup an entirely decentralized web presence”, then why do you want this in the first place? Why do you want to have your ENS name auto-resolve in your browser to a web2 IP address, which (just like the DNS root zone) is ultimately controlled by the Internet Assigned Numbers Authority (IANA), a centralized entity owned by ICANN and tied to the US government?
The main reason that the ENS contenthash record requires one of those protocols (ipfs/arweave/sia/bzz/etc) is because those are decentralized storage solutions. So you’re pointing your decentralized domain name to your decentralized website.