I would happily pay for youtube without ads if I was confident that they wouldn't start putting ads in it anyways like always seems to happen with these services.
Not showing ads is leaving money on the table, and streaming services are like any other company, they will need continuous return on investment no matter what. They will eventually get that return by showing ads[1], shareholders will demand it.
Look at Hulu, Netflix and YouTube. They all show ads now. Netflix previously didn't show ads and now does. Hell, I can't watch a 20-minute YouTube video without watching several minutes of ads.
These services' actual clients are the entities that have million/billion dollar advertising budgets, and not individual customers who pay a paltry monthly fee. Even if services start with no/minimal ads to get their foot in the door, they will eventually have to take their advertising partners' money, and those partners will want to show ads.
[1] https://digiday.com/future-of-tv/ad-supported-streaming-serv...