Not everyone has $$$ to spend on a subscription service and they would locked out of content. Ads are necessary to promote accessibility of content in my opinion.
They could just pirate content, as many have always done. But I see your point, ads can be an acceptable form of payment. I have seen this in a mobile game for example, where you can buy the in-game currency with real money, and you can also get some if you watch ads, fill surveys etc. I'm sure it's abused to hell and back, with telemetry and so on, but I could get on top that right after I ban all other ads.