I don't believe for one single second that this will stay ad free. YouTube will eventually do like all the assholes do and have a tiered system where the first tier is simply less ads not none.
Even when you aren't seeing ads their algorithm is still controlling your front page, allowing them to push partner content that isn't directly advertising but still acts like it. The differences between a commercial for Doritos and an episode of Good Mythical Morning titled "Trying Every Doritos Flavor" from the perspective of the PepsiCo marketing department are that people might willingly click on the GMM video and they probably didn't even have to pay anyone for the video to happen.
Sure Rhett & Link may not have a partnership with Pepsi and are just innocently making content to give their audience (I genuinely believe this), so they've got no part in this becoming advertising, but you would have to be incredibly naïve to believe that Google's algorithm isn't smart enough to recognize that video and others like it as marketable content the promotion of which can be sold to PepsiCo.
Premium subscribers may not be seeing ads, but they are absolutely still seeing advertising.
That's a good point, but to be honest even if those thumbnails were just straight up ads I'd be ok with that. I can simply not click on it. Having any kind of actual ad on a premium service is absolute bullshit though, that's straight up greed...
Hell as a free user I wouldn't mind in the slightest having their segments: videos, segment for shorts, more videos, be broken up to have videos, ad banner, shorts, videos, and banner etc... don't lock me into having to watch an ad but I do understand they need ad revenue and I don't mind seeing the still image ads "of the past" (well past for me since I've used an ad blocker for-absolutly-ever now.)