I’m not sure I fully understand your comment (databases that are special-purpose and evolve out of a sweet spot, or special-purpose systems using databases in worst-case ways?).

I certainly wouldn’t say ES is the former. We use it for some conplex things that (AFAIK) no other (publicly available; I don’t what eg Twitter or Google has going on) system could provide at the scale we need. Everything we’re doing is well within the realm of what ES is built for, and it’s the only system built for it. It’s not perfect, but most of our performance issues could be solved by scaling out, where query or index optimization isn’t tractable.