People aren't normally expected to pay the running costs of doing their job. For example, if a minister travels on official business you'd expect their department to pay for their travel costs regardless of their personal wealth. Why should the King be different? We don't expect Rishi Sunak, who's very nearly as rich, to pay for the costs of Downing Street and its staff out of his own pocket. The Sovereign Grant is actually the budget for running a department of the state. It is not a payment into the King's own pocket for him to use for his own purposes. If the monarchy were to be replaced by an elected president would you expect them to pay for all the costs of official business themselves?

Neither does it come out of your or my taxes. It comes out of the income from property owned by the state. State spending runs at over £3 billion pounds a day, and abolishing the Sovereign Grant would pay for less than an hour of that. That's not going to bring down your or my taxes.