Hello Matt,
We have multiple products that depend on Chromium (//net, //base, //crypto, //content, etc), and in 4 years we have had no single breaking change that couldn't be amended in a couple of minutes. Besides, our projects live inside src, rather than extracting anything from there. This allows us to sync in a daily basis. So right there, more than half of your objections can be dismissed.
I understand you guys have this amazing product and would rather not be bothered by people trying to repurpose it, but the value of this code is not only the code itself. It is a fast evolving code. Besides, all the tools provided to use/build the chromium code outweigh any costs keeping up with API changes.
On a side note... I actually like the fact the code is not kept as a public library. I looked into absail and I noticed it has no stringprintf, because the code wants to be a external-library material, so it has nothing that could be tricky. I'm not a big fan to be taken by the hand each step of the way.
Cheers,