your data is stored on the blockchain, your applications are running as perpetual smart contracts, and you can access it from anywhere in the world with just your private key
It wouldn’t. Author is wrong, Sia doesn’t use the blockchain to store data, just to register storage contracts and mediate storage fees. Sia hosts store O(Pb) of data on servers around the world.
When this article states "If Urbit were reimagined in 2021, it would be running on Sia or Ethereum" I immediately thought, "Wait, you can use Ethereum for decentralized storage?" I'm not following the blockchain space closely, I confess, but I hadn't heard that.
And, Ethereum's web site has a page called "Decentralized Storage" that talks about how it can be used as one! Except that said page goes on to say, "When it comes to large amounts of data, that isn't what Ethereum was designed for." Well. Basically, any blockchain system that requires every copy of the blockchain to be complete is not going to be really great for shoving hundreds of gigabytes -- let alone terabytes -- of data into.
Sia -- which I hadn't of before -- seems like it's explicitly positioning itself as a blockchain-based decentralized storage provider, and is closer to what the article suggests Urbit would be like if it were reimagined now. I don't think Sia's web site does a particularly good job of explaining (or selling) it, but, well, neither does Urbit's.