The day when effective altruist donors significantly crowd out the arts is a long, long way off.
Moreover, I’ve never met a donor influenced by effective altruism who said they would have donated to the arts otherwise. My guess is the money influenced by effective altruism mostly wouldn’t have been donated otherwise, because donors influenced by effective altruism give far more money to charity than people normally do. And if the money came from another cause, it wouldn’t have been arts and culture.
2) If effective altruism became mainstream, effective altruist donors would start donating to the arts
People within the effective altruism community don’t think the arts have no value. They just think they’re not the top priority right now for your donations or your career (unless you’re very unusually talented), insofar as you want to make other peoples’ lives better off.
If a much larger of fraction of people, however, were influenced by effective altruism, then first, we’d solve many global problems, and then priorities would change. Donors influenced by effective altruism would start giving to the arts.
Setting up a foundation focused on the arts in reaction to effective altruism is premature by many, many years. And when it does get to that point, we’ll be behind you.
For a much more nuanced discussion of how people in the effective altruism community think about the arts, see this interview with GiveWell.