If they only cared about performance then they would've left the write concern defaults to not acknowledge writes either locally or within a replica set. Or just read from the nearest replica and don't worry about potential consistency issues.
Also this isn't 2011. MongoDB is not a competitor to Oracle and never really has been by people that knew that a DocumentDB was not usable as a SQL one. It's other SQL databases that are the real competitors e.g. Snowflake, Redshift are.
You know it, I know it, MDB knows it, and most of their customers know it, but that doesn't matter: the stock market doesn't. MDB wants to be valued like a durable-database company, and to be able to charge durable-database prices. They need a plausible durable-database story to get those, regardless of what actual current users want.
It is possible there are still potential users not buying until they get that story. MDB wants those users.
Also this isn't 2011. MongoDB is not a competitor to Oracle and never really has been by people that knew that a DocumentDB was not usable as a SQL one. It's other SQL databases that are the real competitors e.g. Snowflake, Redshift are.