They're not a charity, but their business model is based on ads and they rely on the network effect to support that model. People who can't see anything at all will immediately defect. No revenue at all from the moment that policy is implemented, plus significant risk that enough will leave to weaken the network effect. People who are forced to watch ads with their content will grumble, some of them might eventually defect, but for the most part they will stay and continue generating revenue. Less immediate impact to the bottom line, and hardly any risk of undermining the network effect.
The "just go away" approach is proving suicidal for online newspapers and magazines. It would be no less so for YouTube. Their content is neither compelling enough nor exclusive enough for that to work.
How could YouTube know you won't ever convert beforehand, and at scale? If they started blocking users for suspicion of not buying anything from ads they would have to block a sizeable portion of their traffic and that would have a negative impact on their company.
The "just go away" approach is proving suicidal for online newspapers and magazines. It would be no less so for YouTube. Their content is neither compelling enough nor exclusive enough for that to work.