Economics talks about “price elasticity of demand”. A good or service is elastic if small changes in price greatly affect the demand. An inelastic good is, at its extreme, one for which people would pay literally anything.
Basically YouTube is betting that their Premium subscription is so important, or will become so important, that it will pretty much become a necessity. Likely because of discounts (invite a friend and get a discount) and network effects (YouTube Premium becomes a new video-oriented social medium like Tiktok?) or because the standard free version will become so bad that you will pay the subscription to have a somewhat decent experience.