@Your edit. (I don't think there's a nesting-limit, but a minimum delay before you can reply.)

That makes a bit more sense. I would challenge your phrases "realistic scenarios," "assumed it wouldn't scale," and "overlapping but not identical set of fields," but the real problem we're facing in this thread is all our scenarios are hypothetical and simplified. The fact is, indexing decisions are highly dependent on the exact scenario and since we have different contexts the best we could do is banter on about the "most accurate" hypothetical scenario, which is nonsense.

So I guess my conclusions would be "highly selective partial indexes could save you a bunch of write io" and "otherwise, It Depends™ because Indexes Are Hard®". In any case, your article is a decent introduction to partial indexes in general.