That's a bit disingenuous considering the scalability and concentration of capital compared to labour. Regardless your point doesn't refute mine. Supplying capital reqires zero personal effort.
Supplying capital is compensated by the market because it's a scarce factor of production. It is a scarce factor of production because (i) there are risks associated with its provision, (ii) there's an opportunity cost. Nobody would invest in a startup if there wasn't an expectation of an ROI. Because it's risky, that ROI is high. The ROI is high because the cost base is deflated by the market. This is a risk premium, and it's built into the cost base by supply and demand. This basic mechanism underpins all capital markets and is the reason capital markets function at all.
Your statement about the scalability of capital is of course true. Capital offers the owner intense amounts of leverage and scale. The same can in general not be said about labor except in domains where that labor is scaled by technology (e.g. dev), media (e.g. instagram influencer), or being in a managerial position (e.g. decisions impact 1000-person org), which probably covers about 2% of the workforce.
On index funds, the reason they are profitable to hold is because of the risk premium embedded in its current price, and this risk premium comes about because of the risk aversion of the average investor. As time passes and the world hasn't blown up yet, the price goes up, as it should. It has nothing to do with effort, it is about risk and opportunity cost. It's the same mechanism underlying the yield spread between corporate bonds and government bonds.