I think there are a few factors that would make this idea unworkable. There are two categories of issues, technical and economic, that prevent this from working

I think there are a few factors that would make this idea unworkable. There are two categories of issues, technical and economic, that prevent this from working. I'll go into more detail about the technical issues. The top-level problem is fan-out. If you want to fan the query to the top million domains (far too few to match Google's retrieval depth, but enough to demonstrate the issue), you're going to need to implement some sort of multi-level fanout, since just one machine can't send enough HTTP requests -- nor even establish that many connections -- in a reasonable time-frame. There are going to be severe tail latency issues that will prevent you from gathering documents from potentially relevant sites. You will have to make frustrating tradeoffs about when to time-out per domain queries to provide a good user experience. And many more issues besides. All decisions that are unnecessary if you control the index. Also, internet bandwidth isn't cheap, and you're going to need a lot of it just to consume the top ten results from a million sites. The next technical issue is that the inverted index is only a small part of what goes into information retrieval. Scoring is at least as important. Modern scorers are multi-level, meaning they do one pass over many documents on a simple, correlated representation of the data. Then they do a second pass on a more comprehensive form (i.e. the whole document, and metadata about the document), but over fewer documents. There can even be third and fourth passes. The data for the first pass is often embedded directly into the index, and it would be challenging to come to any kind of agreement among stakeholders about what data should be embedded. This goes double for the second and subsequent passes. Moreover, those second and subsequent passes often use data about the document, rather than data in the document. Data a site owner would be unable to provide or even incentivized to falsify. Not less than these problems is the issue of where to run the scorer code. If you're running it locally, you're operationally 90% of the way to the complexity of an inverted index. Why not go all the way? Then of course there are the economic issues, which, roughly, are: "Why should I pay all this money to host an index of my site that nobody uses when Google will do it for free and charge me nothing?"