But what about the creator's expectation to get paid for their hard-earned views, to then pay for the gear purchased and the production of the entire video, including team members?
If you're really anti ads, don't take the content for free, find another piece of content that answers your ad-free philosophy.
YouTube has every right to create a proprietary YouTube client (which they do on mobile devices) and prevent browsers from accessing it. Don't serve files to my browser if you don't want me to use them.
Creators, for better or worse, are putting themselves at the mercy of YouTube, as serfs to feudal lords. I'm sympathetic, and happy to pay (and do) for your goods directly, but don't complain to me if your lord mismanages your affairs.
Even TV users had VCRs to save content "offline".
Definitely not illegal to do so, so do as you please, but don't turn yourself into a white knight by some mental gymnastics with the sementics behind what a user agent is. You want to enjoy the video without ads, regardless of the consequences on the creator who made that video — and accessorily, the service hosting it and streaming it to you!
Google is in the process of breaking this deal themselves - they're now adding ads to videos which users did not opt to monetize, and they're keeping 100% of the revenue. Taking the content for free, if you will.
Remember, forcing you to watch ads means it's not free - it's you trading irreplaceable moments of your life for something. Unless your time is worth absolutely nothing, ads are extremely expensive to you.
Try running a hosting platform yourself and you'll quickly see just how crazy expensive bandwidth is. It's just ridiculous to expect that someone will 100% subsidize your video bandwidth for free.
This is begging the question that hosting non-monetized content is actually a net loss for YT. I'm saying it's actually not, instead the aggregate volume of free content keeps viewers on the site longer, and the higher viewership numbers result in a significant net profit.
More likely, YT decided that this arrangement, while profitable, wasn't making enough money for them, and because they must show their investors ever-increasing profit margins they decided to mildly poison their user experience in exchange for higher profits.