The argument is about government obligations in the present. The curator doesn’t need to assume that more subsidies would have been helpful in the past to conclude that governments must provide subsidies in the present.

Too strong. The conclusion is that governments must help support the arts, not that they must be the only source of support. Even if government and private support are both necessary to sustain the arts, the argument still stands.

Irrelevant. The premises already establish that contemporary societies shouldn’t be turning to aristocracies and religious institutions to help the arts. So whether or not those sources would help the arts has no impact on the argument.

The only support for why contemporary societies should worry about financing the fine arts is an obligation to be stewards of cultural heritage. The curator must assume that this obligation involves maintaining the fine arts. Otherwise, the argument gives no reason for taking action to help the arts.

Too strong. In arguing that subsidies are required in the present era, the curator relies on the fact that analogous subsidies were important in some past eras. But there’s no need to assume that such subsidies are required across the entirety of time.