This is a thing that has been done on Twitch for several years now. The 4.99 isn't really a subscription, it is a recurring donation. You might get some things like special emoji or a symbol next to your name but that is it. The purpose is not to be able to view the content, it is to directly support the streamer/YouTuber.
When I was on Twitch streaming the ad revenue just wasn't worth it as most of our people were running ad blockers anyway. We also didn't like showing ads. So we just didn't make money as I really wasn't trying to make a living on it anyway. But some of my friends from that community had a lot of subs and I am sure if I put it on we would have had a bunch as we had some dedicated fans. I just didn't like pandering. Unfortunately this is going to become a thing on YT now, but it is nice to provide support other than ads.