It isn't like that decade has no value anyway. It has still been 10 years of learning aikido, even if OP is underranked. Way better underranked than overranked. Why would OP want to get those ten years back? The next ten years are what's up for grabs.
Absolutely decide based on how the training suits, not on ranking problems. Absolutely check out other dojos, styles, arts, everything. The past should not necessarily remove any degrees of freedom....
The "nearly a decade" is sunk costs. You can't get it back, and you shouldn't base your decision to stay or go on the value of that time. Nor should you base it on the likelihood of promotion, given your messy situation. If you decide to stay, do so because you like the training -- however, even if you do decide to stay, I think it would be wise to investigate other options. If nothing else, it will give you valuable perspective.