I am really confused how it's supposed to be "decentralized" if you have to have a sign-in with a specific provider to use it.

Can you log in if Blockstack-the-company goes down or has a technical issue?

Your identity is on the blockchain and that is decentralized so even in that case you or anyone can run a separate node to handle them. Also all the auth library is open source so anyone can work on that.

Its a distributed database, it doesn't matter if the blockstack company takes a long walk off of a short pier.

It is free and will not take almost no special info, give it a try ;) The Blockstack auth with assure that your data is secure and encrypted

Many things are free to sign up for. Heck, I regularly sign up/join any Show HN-featured site.

But there's zero reason for any site not to offer the tried-and-trusted email and password, even just as as a fallback option, especially these days where trust in online stuff is seemingly at an all-time low.

Google, Facebook, et al., companies with massively larger, dedicated security departments swore up, down, and sideways that "everyone's data" is "secure" and "encrypted" ";)"