Using mongo as a DB adds more risk then using self hosted Postgres or MySQL. You can use provider hosted Postgres or MySQL for less money and a better product.

McDonalds is very popular, doesn’t mean it’s good food. Be wary the metrics of success you use.

Can you quantify anything? How does mongo add more risk? How is mysql a better product. It doesn't seem like you're saying anything.

How can it be your largest expense but running your own db would save only a couple of bucks?

We don't spend much :)

Just because it works doesn’t mean it wasn’t a poor choice still.

You could have said the same thing about storing your passwords in plain text.

Bit of a disingenuous statistic. It’s popularity is down to being the default choice for people learning the MEAN/MERN stack. Which is a large number of people currently teaching themselves web dev and goes hand in hand with the rise in popularity of Javascript. It’s got little to do with wether people need a document store or wether it’s good at that job, 99% of the time it’s being used to dump relational data.