Then, once the big brands come to play, the platforms change what ads are deemed acceptable and start kicking off those that started out using the platform.
You nailed it.
But that totally depends on the brand tolerance for it. For example, if you are a big family brand, you're not going to be an early adopter of Outbrain and Taboola type stuff if your adjacent ads are acai berry drinks, colon cleanse kits, etc. that all link to flogs and fake news sites because the network might be turning a blind eye to quality at that stage of things.
And the way the purging happens is pretty interesting too. Typically it comes in the form of new policies. I forget what the wording was at the time, but I recall when FB started making the shift, some of their policy wording and examples was so obviously targeting recent things affiliates had been doing on the platform it made me laugh. Even reading through their current policy site's prohibited list is pretty much a checklist of every major affiliate category there is.