This isn't entirely true, because they use more than one search index.
replyThe other search indexes are largely negligible in comparison: [0]
reply> This is not a competitive market. It is a monopoly with a distant second place.
> The search index is irreplaceable infrastructure. Building a comparable one from scratch is like building a parallel national railroad. Microsoft spent roughly $100 billion over 20 years on Bing and still holds single-digit share. If Microsoft cannot close the gap, no startup can do it alone.
This is a problem of their own invention.
replyNobody said you have to index the entire web.
The web would probably be a lot healthier if we had several small search engines that focused on niches rather than 5 failed search engines that tried to index everything that was ever written and then ended up paying Bing.