I guess you can get 90% of the way, but the remaining 10% becomes really hard unless you're Google scale. But even several 90% alternatives would be better than absolute monopoly.

Do not forget that Brave (the browser) was designed/marketed as part of the US Culture Wars, basically as a Firefox-but-for-Conservatives.

Brave (the search engine) apparently follows the same strategy. The reason for having this index is basically political. Brave doesn't want to be impacted by Google or Bing's editorial choices. Of course, Brave Search will certainly be doing its own editorial choices.

I think it is definitely a hard problem to solve on a large scale to address latency, quality and size of the index they plan to address. It definitely isn't as easy as spinning up an elastic search cluster.

I agree that getting something "mostly" good or a domain specific search engine isn't as hard with the newest advances in this space with vector similarity indices.

There are tons of hurdles. For example, many major websites will block you if you are not a crawler owned by a few companies. They have to be in Google’s index to survive, but that doesn’t mean they allow everyone else to copy their content.