The selection isn't happening on the publisher's side, but on the ad-tech side.

Every publisher can enjoy the tech provided by Google, and participate in gaining money from this system, relative to it's popularity.

So what we see is that small players within the ad-tech business are dying, while bigger players (Google, Facebook) can not only adapt to the law, but also help writing the law.

So a hypothetical switch to first-party/server side ads won't have a negative effect on small blogs, because they can simply chose Google as their ad partner, which will create the necessary solutions.

The only advantage big publishers have over small publishers is that it's easier to scale the extraction of money from 10 million visitors, compared to 10,000 visitors.