I don't see why leaving the impl packages at the leaf level makes the library better consumable for users, IMO the opposite is the case.

Same here. I cannot really put into any other words.

I firmly believe the only strategy which guarantees the least annoyance is the no-change: too bad we got it "wrong" (even assuming it is) initially.

Here I highly disagree with you Sanne Grinovero. If we were to agree that the opt level split would be the better approach, then it would make to change. Sometimes disruptive changes make sense and are required.

But anyways, initially I just wanted to point out that we even do not do the chosen approach properly. This adds additional confusion imo. At least we should do one approach consistently.