> There's very few legitimate use cases for not using a RDBMS as a main data store
I don't think that's true—all the old cases that were legitimate before are still legitimate. The price of both persistent storage and memory has simply come way down to the point where many computational workloads are viable under vertically scalable databases again. I suspect the pendulum will swing the other way one day yet again (and of course there is a rich ecosystem of ways to query horizontally, if not transactionally, with SQL that will likely temper this swing even further).
The price of persistent storage and memory was the same for Mongo as it was for Postgres back when the NoSQL movement happened. Mongo wasn't made of magic scaling fairies and still needed resources. As soon as you cranked up the safety of Mongo to a nearly acceptable level it's performance fell through the floor. The only reason people though it performed amazingly was because of Mongo's rather deceptive marketing.
I use the term "nearly acceptable" because for a long time Mongo's data consistency was absolutely crap (https://jepsen.io/analyses)
Personally I think people used Mongo because it was new, shiny and exciting and it promised performance (though it didn't really deliver for most people).
I am certainly not going to defend mongo ever having being a good choice; I was referring to other "NoSQL" databases (namely, the large number of highly scalable key-value stores).
But you wouldn’t use a key value data store as your main store. It’s a specialized kind of store. Of course Redis and friends have their place. Just not as a main store.
In addition to changes in ephemeral and persistent memory, the other big difference between now and nosql’s heyday is improvements in distributed relational transactional databases (newsql).
We haven’t exactly circled the square on CAP but we’ve certainly bent it.
I don't think that's true—all the old cases that were legitimate before are still legitimate. The price of both persistent storage and memory has simply come way down to the point where many computational workloads are viable under vertically scalable databases again. I suspect the pendulum will swing the other way one day yet again (and of course there is a rich ecosystem of ways to query horizontally, if not transactionally, with SQL that will likely temper this swing even further).