Warren Buffett says it's OK, and has an excellent explanation for this. If I recall correctly: imagine you take all the investors in the US economy and put them in a room. Divide the room in halves. One side contains all the active investors, the other side contains all the passive investors. If each side owns roughly half of the economy, their returns will be equal. In that case, it's better to sit on the side with the lower fees... so one should naturally sit on the passive side.

Setting Warren Buffett aside, what you neglect to mention is that the stock market is both a primary and secondary capital market. We can speculate about how to speculate... whether to be passive or active... but this concerns only the functionality of the secondary market. There's still primary market functionality: companies issue stock to raise capital, buyback stock, and issue dividends. Thus, even if all the investors are passive, there's still always one active agent in the game: the company itself. And a capitalization-weighted index is ideally suited for this activity: it automatically shifts capital away from companies buying back stock (essentially, companies returning money to investors) to those issuing new stock (essentially, companies seeking to raise capital).