The high cost of productions is severely limiting which operas are

(A) We don't care about the preferences of a FEW opera purchasers

The justification for getting rid of (B) that we don't care whether large corporate sponsors still have a DESIRE to sponsor. We evaluating whether denying them the ABILITY to sponsor would result in the public seeing less famous operas.

I agree that (C) at first feels like cheating ... are we just denying the IF condition from the conclusion?

Not really. The IF condition from the conclusion is that production budgets would be lowered so that box-office and private donations can cover it. Let's say, to make this more concrete, that we only take in $50,000 per show from box-office and private donations.

Fine, then our production budget is set at $50,000. Will we now be staging less famous operas?

No, according to (C). The only operas we could afford without corporate sponsors will be the most famous of operas. So, according to (C) our $50,000 budget will only help us to produce the most famous of operas.

What's hard about interpreting this answer choice correctly is how counterintuitive it seems to me: it's essentially implying that the most famous operas are cheaper to produce than less famous operas are.

(I tend to assume the opposite, even though there's no reason more famous has to mean more expensive.)

As a more general rule of thumb, if you're trying to weaken an argument that's expressed in conditional form ...
IF we do this thing, THEN we get this result

... the test will almost always weaken it with an answer that shows "even if you do this thing, you still WON'T get the result you're seeking".

So I was reading answers looking for, "Which one tells me that, following our plan, we WON'T get less famous operas?"

Seen in this light, only (C) is relevant. None of the others even discuss how famous the operas will be.

(D) We don't care whether large corporate sponsors will or will not support opera productions. The conclusion is conditional, IF budgets can be reduced to the point where there is no need to rely on large corporate sponsors THEN ...
(E) Makes the same mistake as (D). Even if individuals can't match the donations of corporate sponsors, individuals might be able to cover all costs IF production budgets are reduced.