"What will happen to the web when uBlock Origin becomes prevelent enough?"

We already know the answer to this because we saw it in the days of the early web: advertisements on web pages will be pictures and/or text served from the exact same FQDN as the website itself.

The problem has never been how to serve ads - it's trivially easy to insert an <img src> and perhaps size it randomly, etc. Or you can just put in a text block.

The hard part is the ad networks and the user tracking and the analytics. These are the parts that are difficult to self-host or self-provide. Their absence will cause ads to be relatively less valuable and less user-hostile.