I think that we're looking at an alternative funding option for content creators/producers/webmasters, etc...There will always be room for ads. There will always be a group of people who simply do not want to pay for the content they're consuming and so they pay by having their information sold or their eyeballs bombarded with ads. To that end, YouTube and ad based services won't go away.
Currently, Coil is a "Feel Good Subscription" and provides zero competition with YouTube's revenue model. For YouTube to feel the pain it would require a whole helluva lot of content creators to abandon their platform entirely and host their videos elsewhere which would ultimately cost creators by having to foot the bill for the servers. The value for creators in YouTube is in the hosting and the ubiquity of the platform.
The question you're really asking is, "Will creators abandon YouTube as a platform because Web Monetization provides a better experience than YouTube's current services for content creators?" But, I'm not sure you can compete with free (hosting, ubiquity, etc). That being said, I COULD potentially see it taking a chunk out of YouTube's premium subscriptions because that's essentially what Web Monetization is (a premium). It's difficult to contend with free, and these days, you need people to stumble across your content and the money is made elsewhere.
Platform's such as Patreon provide an alternative income stream to some creators who provide an ad-free experience or that individuals want to support despite being able to consume their content freely at the cost of time and attention (ads). But even so, the creators are providing "exclusive" experiences to subscribers.
I think Coil will provide an alternative funding option for creators but it won't supplant advertising simply because creators must reel people in somehow, and offering a base level of content for free does a good job of that.
Coil MIGHT supplant the premium experience, but design and time will tell. In the long run I certainly hope so.