The problem here is the word "Always". Encryption is good for just the reasons they say. But only encryption, always encryption, not having an option for plain text is highly centralizing in itself. This is because the current status quo for encryption is to use TLS based on certificate authorities. And CAs are always highly centralized and highly centralizing.
If Lets Encrypt ever goes corrupt like dot Org did it would cause an incredible amount of trouble and that entity would have power over a large portion of the web, if not the entire internet. There's an easy solution to this though. Don't throw alway plain protocls. Plain and TLS wrapped are synergistic. Use both. There's no need for, and it is damaging, to always encrypt without an option for plain text.
A hypothetical downgrade attack is not an excuse for using only highly centralized TLS CA based protocols in this context.