Not the person you have asked but at work (we are a CRM platform) we allow our clients to arbitrarily query their userbase to find matching users for marketing campaigns (email, sms, whatsapp). These campaigns can some times target a few hundred thousand people. We are on a really ancient version of ES, but it sucks at this job in terms of throughput. Some experimenting with bigquery indicates it is so much better at mass exporting.

Fair; my question was mostly in the context of ANN, since that was the discussion point - I have to assume ES (as a search engine) would not necessarily be the right tool for data warehousing types of workloads.