(I think my previous questions were not clear or comprehensive enough as I always get negative rating or incomplete answers. So let me try again, as this question is really important to me.)
I truly love my job, believe in the company, invested and sacrificed a lot in the projects that have shaped it to become what it is today. However, some new organizational changes have led to me losing most of my managerial responsibilities, and a person closer to senior management has been groomed (and now confirmed) to become head of my team.
I am already excluded from strategically important meetings, am given tasks without knowing the rationale and generally am less involved in decision-making.
As I have been a top performer and always got only positive feedback in my reviews since I joined the company as a junior manager, I believe the reasons for this are political rather than my own performance or skills.
In any case, I would find it sad to leave. On the one hand, I don't want to work with less responsibilities because my ultimate goal is to gain experience towards senior management. On the other hand, I really believe in the potential of my company, I know it is growing fast and what we have done so far is nearly unprecedented in business. So, I have mixed feelings about leaving.
As my future is uncertain (not sure if I could be made redundant; not sure if I will ever get further opportunities to grow; not sure what the next reorganization could mean for me; very unlikely to grow salary, etc.) I am struggling to find good practical/non-spiritual reasons to stay, but I feel that if I did stay and all went well, I would feel more fulfilled in terms of witnessing how everything I started evolves and see how the company fares.
So... in my situation, what are the advantages of staying rather than quitting?