They have a social contract with the creators: you upload content for free and users will watch content for free. If they went fully paid creators and viewers would go somewhere else in Ain instant.
I don't think that quite represents the existing social contract with most creators. I think the practical contract is in effect "I will upload content and you will compensate me fairly for it". If a subscription-only model led to increased content creator payouts I don't think you'd see a massive exodus from the platform. Regardless, I'm sure it's in the ToS and all that jazz that any of the terms can be changed at any notice and that your only recourse is to yank your videos and take them elsewhere if you don't like how things are going.
And, interestingly, the biggest exodus I've seen hasn't been to another free platform, it's been to a subscription-only platform (CuriosityStream). The documentary-style YouTubers didn't think they were making enough money on YouTube because of how hard it is to make their content vs your typical vlogging stuff, and they know their viewers are willing to pay more to access it, so they made a pay-only streaming site to earn more money from their product.
And, interestingly, the biggest exodus I've seen hasn't been to another free platform, it's been to a subscription-only platform (CuriosityStream). The documentary-style YouTubers didn't think they were making enough money on YouTube because of how hard it is to make their content vs your typical vlogging stuff, and they know their viewers are willing to pay more to access it, so they made a pay-only streaming site to earn more money from their product.