I recently interviewed at a startup that explicitly uses both, with PG being something of an authoritative backend store that feeds into ES as the primary frontend store (since the frontend is search-heavy).

It might be possible to get it to work without PG, but I imagine it would require a huge engineering effort (akin to an entire product on its own, which, IIRC, is what CrateDB is).

(Although I believe I understand your point, I fear that your tone and ad-hominem language detracts from it).