My argument is that a hypothetical compromise of an app never gives google more power than they could hypothetically have now. I also don't see why it would be harder to detect. Why do you think that's the case?
Even if one OS component (Google Services) is centrally controlled and can be used to attack you, this does not mean, that you should make other parts less secure. Real-world attacks are complex and backdoors are fragile and prone to being detected. Embedding a backdoor in proprietary code of Google Services is easier than embedding it into AOSP. Hijacking a specific application is easier yet.