I absolutely don't doubt a document store including mongo is an ideal solution for certain specific applications. I just haven't come across one yet personally. For me personally, I would use Postgres for most things and JSONB columns for document-like things until I found a reason to change.
I've personally never had to shard anything in my professional career. Although IDK how painstaking the sharding would really be for most uses I can imagine besides like, social media. Facebook has to deal with everyone interacting with everyone. But the vast majority of situations most developers see isn't like that. You can run Salesforce purely sharding at the tenant level, since I only need to see my tenant's data.
Anyway I feel the need to be emphatic, I'm sure Mongo's benefits can be realized somewhere, just not in any small or medium sized company that doesn't need sharding, and does need a schema.