> The reason DBs like Mongo or Dynamo exist is because
Not quite. The reason "DBs" like those exist is purely due to fashion. Lets not kid ourselves into thinking they do anything better, save the exception of making data hard to access, which might be a project goal in some cases.
Dynamo definitely scales better than anything else at the tradeoff of not guaranteeing durability in the case of enough node failures and (like most distributed databases) not allowing interaction between different pieces of data.
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