maybe you could pull down changes on a daily basis? not a complete reindex but just updates that prune dead links and add new ones.

the problem is this would be completely useless for current events. but querying sites you use regularly, this could be an option. I often end a google search with modifiers like "wikipedia" or "reddit" or "stackoverflow". If I could store indexes to those sites offline that might be useful.

The creator of this offline search could potentially make money by indexing shopping sites like Amazon, eBay, etc with affiliate links.