It is possible to scale databases Open Source (SQL and Nosql) as much as a commercial version.
An example of this is the Mariadb, replacing Mysql for high scalability and performance applications.
In addition, giant companies use Nosql or even SQL databases in clusters to provide the necessary data.
What usually differentiates a commercial solution is the amount of features offered, which are immensely larger in banks like Oracle and SQL Server than in Mysql or Postgresql, for example.
In addition, companies like Microsoft and Oracle offer a stack of integrated solutions, while in solutions Open Source you would have to configure and integrate everything "manually", generating sometimes higher costs with lower benefits.
Remember, for companies it is all about cost-effectiveness. And it also involves the issue of having a "safety" that the product will work. Then we come to the question of technical support.
For a company, a free system that doesn’t work is more expensive than a paid system that works. When someone pays for an Oracle or SQL Server license he is buying the warranty of these large companies that bugs will be promptly corrected and your doubts will be resolved quickly, according to SLA set forth in contract.
Note, however, that this is common for "normal" development companies. IT giants like Google and others, who often create and support new technologies, can afford to use only Open Source because they have the manpower and resources to create their own infrastructure and support all of that.
At this point, it is to their advantage to have a super specialized engineer to deal with the problems of the tools within the company itself. For most businesses, however, this is not an option.
Good question, here at the company we use everything open-source
– Jorge B.