I doubt many publicly owned buildings generate income, that is not really what most state activities are intended for. Nevertheless the royal residences earned nearly £50m from tourism in 2019-20.

Buck House is mainly an administrative building for the head of state, with offices and function rooms. Come the revolution you can sell it off, and it might raise enough to pay for the NHS for a day or two. Until then it is the responsibility of the state. It is not the King's roof, it is the government's, and they are giving him the money to fix it. An elected head of state will still need an official residence and an administration.

The role of head of state, whether a hereditary monarch or elected president, is a function of government to be paid for from the public purse, which is funded by taxation and income from revenue-producing activities, including the Crown Estate. It is legitimate to question those costs, and in a democracy to campaign for change. However it is a nonsense to claim that this is "stealing" from us, any more than the costs of running other state functions such as 10 Downing Street, the Foreign Office or indeed your local library are stealing from us.

In the context of the public finances £125m is a rounding error.