Well, you might start off down that path, but eventually you'd find that if you try to execute analytics via PostgreSQL via FDW via Multicorn via MongoDB, you're only able to push conjunctions of simple relational operators on original (non-derived) fields in the source collection.

What that means is virtually any query will end up executing (via PostgreSQL via FDW via Multicorn via MongoDB) by first pulling out all (!) the data from all (!) source collections, relocating it to MongoDB, and then executing the query. Possibly, in fact, these full collection scans might be repeated multiple times, especially for nested data, crosses, and other types of operations.

And then you'd decide that "solid engineering decision" wasn't so solid after all. Then hopefully you'd quit MongoDB and go work on PostgreSQL full time. ;-)