Even an index fund has some human-curated criteria for what to include, though, right? The S&P 500 isn't open to just anyone. So it seems totally legitimate to have it be not completely algorithmic.

If there were an "Open 500" that was trying to be like the open equivalent to the S&P 500, I would happily donate to it. Right now I do GitHub sponsors but it feels kind of random.

You just don't want to include projects like React or TypeScript that are operated by a for-profit company - they don't need our donations. You want it to be, this money is actually going to an organization that will invest it in software quality.