The nature of the use-case evolved and, at one point, the MongoDB + Mongoose ORM combination was in fact optimal judging from various aspects including tooling familiarity with most to the primary use-case being the managed cloud service. As the situation evolved, it no longer became optimal and was superseded by various stack-wide adjustments.
Regarding tooling familiarity: In the earlier stages of a company, I'd argue it to be important to pick what you can ship with fastest. You don't want to prematurely, for example, setup a complex microservices setup and deployment with K8s for an idea that is in its infancy and that might not even get that much traffic to begin with. Instead, deploying a monolothic app to Heroku might be the optimal choice (YC has a more extreme term for this they call "duct tape engineering"). As the company grows and there is clearer product market fit, it makes sense to further optimize the stack and perform adjustments as needed.
Regarding the use-case changing, it wasn't entirely obvious in the beginning that a lot of users would self-host the product, so it was designed more-so with the cloud product / managed service in mind; in hindsight things seem more obvious but not in the moment. As it became clearer that more and more people were going to self-host this product, the team shipped more features to accommodate for a simpler self-hosting experience and, as part of that, the PostgreSQL migration.
Regarding tooling familiarity: In the earlier stages of a company, I'd argue it to be important to pick what you can ship with fastest. You don't want to prematurely, for example, setup a complex microservices setup and deployment with K8s for an idea that is in its infancy and that might not even get that much traffic to begin with. Instead, deploying a monolothic app to Heroku might be the optimal choice (YC has a more extreme term for this they call "duct tape engineering"). As the company grows and there is clearer product market fit, it makes sense to further optimize the stack and perform adjustments as needed.
Regarding the use-case changing, it wasn't entirely obvious in the beginning that a lot of users would self-host the product, so it was designed more-so with the cloud product / managed service in mind; in hindsight things seem more obvious but not in the moment. As it became clearer that more and more people were going to self-host this product, the team shipped more features to accommodate for a simpler self-hosting experience and, as part of that, the PostgreSQL migration.