It is up for grabs. Die as the victim of a violent crime and guess how much power your estate has to prevent an autopsy. Die of a virulent communicable disease and guess how much power your estate has to prevent your body from being cremated and the pathogens within rendered as inert as possible.

Your body is your possession like your house or your car, and I am sure you can image how often a clear legal directive like a will is placed before a state body like a court and judgement passed.

Objective fact has provided a ton of evidence for materialism and very little for any alternative.

This is not a statement of faith, it is a statement of law. Once you are dead you have no rights and any possessions, claims, and liabilities pass to that legal fiction we call your estate. Perhaps there is some non-Western culture that handles things differently, but other than the estate being more of a familial construct rather than a legal one in some instances I am not aware of any deviations from this of any significance and look forward to being shown a counter-example.