I would say that in many relationships, the submissive has greater power. Personally, that's not the kind of relationship I ever want to be in as a sub/slave. I have been in a relationship for which I made the initial decision to submit but it was the last decision I ever made, no limits, no safe words, no getting out if I was truly a slave, honoring that one and only choice I had at first. The power in that kind of relationship, whether you regard it as given or taken, belongs to the Master.
Perhaps we do have to accept the notion that everybody's truth is right because there are so many ways a relationship's dynamic can go. However, prior to his/her total surrender, the power is in both their hands until that exchange of power is made. Then it becomes the Dom's/Master's benefit, along with the authority, control and final say in any decision. If the submissive retains any power, he or she is topping from the bottom. So, if we are examining a working, successful D/s relationship, the power becomes that of the Dominant or else it's a sham as D/s relationships go, in my opinion.
If the submissive reclaims the power, say to end the relationship, he or she must concede that they are not doing so as a sub or slave but rather stepping out of that role in order to regain control and make final choices.
If you were talking a TPE, I would agree with your statements totally, but D/s relationships range the gamut. You are assuming that a Ds or even Ms relationship is total, where the dominant/master has total control, where the sub/slave has nothing, and that is a false assumption. I heard these arguments for years about a TPE, that unless the sub cedes total control it isn't real, unless they are willing to accept even physically harmful damage from their M or D, including being killed, and that to me was simply telling others how to live. The thing about a D/s is , or even an Ms, it is negotiated between the two parties, if an M wants total control, including cutting the s off from friends and family, etc, and the s accepts that, then that is their relationship, another M might not want to be that controlling about everything. I agree the power is there and a TPE s who accepts giving over total control is powerful at that point..but the reality is, that many D's and M's don't want that kind of power, in effect they cede power to the sub they wish them to have (whether it is in putting that into the contract, or granting it to them in the relationship, it is still ceding his/her authority). Saying that a s who retains power is topping from the bottom or is a sham IMO is trying to say there is only one true relationship, and that isn't true, power these kind of relationships is shared according to the rules. If the M or D allows their s to have a safe word, it might violate other's views of their relationship but within their world, it is not a sham. If a sub takes on themself something they shouldn't, like let's say punishing another s without being given that right, then yeah, it is topping from the bottom. Exercising power where it isn't granted is topping from the bottom, using the power granted is operating as the M or D wants *shrug*.
I also firmly believe that anyone, D or S, retains the right to walk away from a D/s, M/s relationship if they feel they are at risk, that the D or M is being abusive, that he/she is psycho or otherwise seems to have forgotten their s is a human being, not an inanimate argument. I have seen slaves in these kinds of relationships get brutally beaten by their M's, broken bones, internal injuries (and have seen one schmuck lawyer try to argue that it was okay since they were in a consensual relationship....that lawyer IMO should have been disbarred for even trying to argue that), that there is a line where even TPE's can't or shouldn't go, and that is self preservation...just my opinion, but I would stand by that one until the end of time. I realize that can be a gray area, because a vanilla could say that about a wife who likes to get spanked, but my point is that the s has as their duty to themselves to preserve themselves, that is the one place I think they have to retain power.