Ok let's get back to why it might be useful do define models:
The database needs protections from it's foremost enemy: the developer
If you have some basic layered structure of data you force the developer to think instead of just vomiting data in your database.
If you have at least partial structured data you also have a bullshit filter of what comes into a database.
If you have structured data you can be shure that for example an idex of a field exists. There is a reason why you can set indexes in mongo.
Schema and Data Modeling is part of MongoDB, heck it is even in the documentation: docs.mongodb.com/manual/core/data-...
You could now come again with the argument why not use PG, simple less options querying the unstructured Data within you possible model.
A Mongo Cluster is also working way more reliant as soon as it comes to cross region scale, even if you use a managed service like RDS
You brought up the point it is easier with using DaaS and then brought up some strange views using mono cloud SaaS.
Why not use single Vendor cloud?
How nice that you choose Netflix, roughly one year ago AWS East Us crashed and was down, half in the US Netflix, Github and some other service have not been available for over 6 hours.
Gues what, Netflix moved to Multicloud and uses Google Cloud as well, because of this.
Also cloud platforms, regardless of form factor or delivery method, are not all interoperable with each other or with legacy systems. There is often an assumption that all will play and talk nicely together and push and pull data easily; this is not the case. Therefore, the ability to mix and choose between offerings in a multi-cloud environment
You cannot easily move your infrastructure to annother provider. The question might be why would i? You do not know exactly what is on layer below, there might come changes that you did not antacipate. Or simply a matter of cost.
Additional you have advantages like lower lateny, higher security.
Most of the bigger companies, are or currently moving to multicloud enviroment.
I drop out of this discussion now, because in my opinion the view of this discussion is only in seeking to find flaws.
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