US copyright law has teeth for stuff like this.

If the RIAA goes after the OP for statutory damages, he's basically fucked for life. And they love to make examples of people. Did everybody forget Kazaa and Limewire?

Not remotely on the same level.

Kazaa and Limewire were moneymaking companies with fairly undefensible behavior from a legal perspective. This is an individual using a website the way it’s supposed to be used, for documentation purposes. A judge would “expeditiously” send the RIAA packing with a large bill for defendant’s legal fees.

Right, it's not the same level. Kazaa and Limewire faced theoretical liabilities in the billions.

That doesn't mean that the programmers of youtube-dl, or those who choose to engage in spreading the program, can't be held liable for much lower levels of damages which are still financially ruinous for an individual even if they're small on an absolute basis.

I've been on the other side of an RIAA lawsuit, and their lawyers are aggressive. They offer a carrot settlement, but if that settlement is declined they will beat you with a stick and offer no mercy.

They could call the U.S. attorney and go after the OP with federal felony charges.