Who has the power in a D/s relationship?

Maybe ethically the sub has the power - because veto - but has a worse negotiating position because the dominant is the permitting submission to happen. Definitely, both are morally and legally equal.

However, functionally, all this stuff tends to collapse over time and people just get used to the power relationship.

There are built in human mechanisms for this, including the Authority Ranking Relational Model, which is basically a relationship mode humans naturally slip into, and Operand Conditioning, which is what it sounds like.

Also, whatever problems power exchange solved will reappear if the power exchange goes away, so both parties have an interest in maintaining the dynamic.

Which means, ultimately, the power rests with the dominant, which is surely the point of the exercise?