Currently you can block many ads based on their url (which is from a third party provider). Using umatrix, ad and tracking is often clearly marked in red, and the corresponding js never even loaded. I don't think that would change with webasm?
That's possible now. You can serve the ad so its indistinguishable from any other image. Redirect it and clicks through your server. Give it an obfuscated DOM path.
Its just a hassle and can't be done with pasting a line of code
Pretty off-topic, but the reason why it isn't done is tracking, not ease of use.
Ad-networks want the ads pulled from their servers so they can track views. If web-owners serve the ads themselves, then the ad-network must trust whatever the web owner says about the numbers of visits/clicks/etc.
For now the ad-networks just don't care about ad blockers, because they are making money hand over fist anyway. If things ever get hairy for them, I suspect they'll switch to a reverse-proxy model. You point your domain to their servers as you do with cloudflare, and they they serve your content with ads injected in the right places, served under the same domain. This would be pretty easy for web-owners and completely nullify ad-blockers in their current incarnation.
The more important reason this isn't being done more often is that advertisers want to push their own code and make connections to their own servers in order to protect themselves against ad fraud, and because they want their own tracking data.
