In other words, the set of github core services has expanded because you don't use third party tooling for some of those services anymore.

For us, yes - and likely for a lot of other users. I'm not certain who else has dealt with the headache of being migrated off their legacy pricing plan but it ends up pushing those internal offerings a lot harder than the old approach did so if they're seeing successful conversions it's likely they're seeing significantly more load from mature codebases with expensive CI/CD pipelines.