"borderline illegal"
Something's either illegal or it's not. As an American, I believe I am within my rights to capture whatever traffic can be legally sent to me at any instant.
Well, it's illegal then, American or not.
YouTube license is pretty clear with that: no download allowed (unless YouTube offers you a download feature for the specified video). Also, no add blocker allowed.
As stated, I'm not a lawyer and work off what I know, but I believe you may be confusing a civil agreement and the breaking of a law.
Licenses list terms of service, upon the breach of which (unless it is a signed, notarized document detailing the civil promises made to a body, upon the breach of which no law has been broken, yet you have violated a civil agreement and can be sued and be required to pay damages or, if applicable, submit to some other criteria) all YouTube can really do is deny you access to their services, unless you have also committed some other crime against them.
The capturing and keeping of packets legally sent to you for personal use I do not believe is illegal in the same way capturing and keeping television (formerly signals now digital data) content for personal use. It's the reason, as I said, to my knowledge, you can tape a radio station and use the recording for your own personal uses (defining personal uses as, consuming the content again without redistribution).
Terms of services are written as legal documents, but their effect as contracts is dubious at best and completely worthless at worst. They are intended to set terms they intend you to follow for continued use of their services (hence why it's called terms of service) and often include details on liability of said services, for example as a way to absolve their liability should you do anything illegal on their sites, so long as they actively seek to remove that content.
Again, this is the US law as I know it. You're from Romania, so the laws and extent of unsigned, and often unread civil agreements may be different, but there's a VERY important distinction between civil and criminal courts, and just because YouTube tells you not to download their videos, doesn't mean is a law nor legally binding in any way. They can tell you not to breathe too, but that's not likely to stand up in any court either.