Even for slightly larger organizations, I'd wager the extra work required to procure you're own agreements is a significant expense. Local newspapers (for example) often run on razor thin margins.
While it's true that print newspapers also had to obtain their own ad agreements, they had a lot of other revenue sources back then too. Classifieds were a huge one; another was that issues weren't free for users. Hence how they're struggling.
Podcasts are another medium that require bespoke advertising agreements. And smaller creators have found a solution—combine to form larger "podcast networks" that collectively handle advertising agreements.
This seems to be working, but it also runs counter to your goal of a more decentralized internet. As I see it, the logical end-game in the podcast space is for networks to keep combining with each other in order to take advantage of economies of scale. Eventually, the entire Podcasting space will be dominated by a handful of major networks—or, Spotify will just take over everything, and any semblance of podcasting as an open platform will disappear.
Good points, and interesting.
Hopefully an equally interesting counterpoint:
Publishing and distribution today is a fraction of the cost of paper based media.
When I wrote "small sites" I wasn't thinking about single person blogs but rather about smaller newspapers and similar.
As far as I know there was a golden age for technical blogging but today I personally wouldn't even try to earn money that way.