Actually while not preferred, yes altering metadata is possible by either linking tokenUri to off-chain http server or having the ERC-721 token's smart contract's functions modify state and tokenUri returning some base64-encoded dynamic data based on the on-chain state (which I personally implemented and works pretty great).
Though it brings a less-technological yet more philosophical question about NFTs and what they represent being forever the same/frozen, or not, though I don't think it's the discussion for here.
Hm yeah I think it's an interesting concept, perhaps it's not preferred under current NFT definitions to my mind something can be both non-fungible and mutable. I can own a house, but I can renovate the interior, add rooms, paint walls etc - those changes are done _to_ the house, it's still the same house, and I still own it.
Off-chain solutions where longevity is concerned don't sit well with me - I don't personally trust oracles to be available for eternity or remain benevolent. Your approach based on a smart contract sounds good as it's on-chain. But would it be preferred to have a dynamic / mutable NFT standard with this capability baked in somehow?
Anyway not really my wheelhouse, and as you say probably off topic for now. Just something I find interesting to consider.
The functionality that alters the metadata is actually a requirement for our project. As time passes the token turns into something different. (For clarification: this is clearly defined in our whitepaper, website, Discord server, and most importantly easily verifiable on contract code itself so no shady business here)
Strictly speaking, ERC-721 (the official standard for NFTs on Ethereum) doesn't say that whatever is returned from tokenUri function (which defines all the properties, name, image etc. of the particular NFT) can't change. So in simpler terms the contract can change the tokenUri afterwards (of course if there is code in the contract to support that).
It boils down to how users/community perceive NFTs, and your house analogy is perfect for what we are doing (in our case something is "happening to the house as time passes" but you get the idea). In the technical sense the standard is agnostic about tokenUri mutability, as long as the token DOES have a tokenUri (which can just be a base64 "data:" URI anyway).
It will be interesting to see what's possible, I'm pretty much sure that the "killer app" based on ERC-721's haven't been invented and when this Ponzi-like ridiculously pricing of non-functional NFTs mania is over, we'll start seeing some real solutions.