Earlier quoted context omitted.
> What incentive would Google have to continue populating that index? Presumably they would still want to run google.com and make money off of it. > Would I be breaking the law if I independently crawled and hosted an index without publishing an API for it? No. You would not get the advantage that Google gets when it crawls the web and so would not have access to a large amount of data that nobody else has access to.…
Do any site operators actually block non-Google search engine crawlers because being listed DDG/Bing/etc isn't worth the extra cost of serving the crawler? It sound a bit ridiculous unless they actually don't want to be found. Maybe they only allow GoogleBot because that's all they thought of and the extra cost is in researching what all the other search engines call theirs. Perhaps other search engines should spoof…
The benefit varies with the quality of the search engines, and that will vary between search engines, but it does get larger the more a search engine is used, so a cost/benefits analysis may show Google and a few other large ones are the only ones worth supporting.