If you want to believe the monarchy are a self-interested out of touch family seeking to benefit from their privileged position and legal status there may be little that can be said to dissuade you.
It is worth noting Prince Charles (now king) voluntarily pays tax on the money taken from the Duchy.
More objectively - irrespective of whether the monarchy is politically or socially desirable, if arguments are reduced to a simple financial and tax issue, there are other considerations:
- the extent to which the Monarchy funds the preservation of national assets which would otherwise not be commercially justifiable, and may anyway require public funding
- the positive impact on the economy of tourism and supporting the UK reputation benefitting international influence and business development
Paying inheritance tax on Williams enhanced estate - could the properties be sold off, if so to who, would they be maintained in future for the benefit of the public, does William who anyway will lead a privileged life benefit from the his "ownership".
I am not convinced there is any merit, save the superficial perceptions. A common view often used by the aristocracy to justify their wealth - we are merely holding it in trust for future generations. I suspect this is a reasonable proposition in this case.
Whilst I am personally somewhat neutral about the monarchy as an institution, I would support the status quo.