The point about the URI changing is interesting. One could imagine the current URL stored in metadata and then "virtual" DNS lookups could be done over time/across chains well into the future.
Given the adoption and direction crypto is going with decentralization, we can imagine some new/future incarnation of the technology to exist, possibly on new mediums.
In short, blockchains don't die. They may become zombies, but they never die. Tokens and profit keep them alive; without, they just go dormant.
Therefore, I would suggest a blockchain called Arweave that delivers the permanence for your website:
"Arweave is a new type of storage that backs data with sustainable and perpetual endowments, allowing users and developers to truly store data forever – for the very first time.
As a collectively owned hard drive that never forgets, Arweave allows us to remember and preserve valuable information, apps, and history indefinitely. By preserving history, it prevents others from rewriting it."