You do have the impedance mismatch to OO languages, which are probably used in 90% of all applications. Farter more, I don't really think that all systems have

You do have the impedance mismatch to OO languages, which are probably used in 90% of all applications. Farter more, I don't really think that all systems have relational data, it's just possible to store it in that format. Sometimes 3 document types are easier to handle from code than 30-60 tables.