Performance was not the selling point of relational databases in the first place. It is hard to beat the performance of a perfectly tuned hierarchical or graph

Performance was not the selling point of relational databases in the first place. It is hard to beat the performance of a perfectly tuned hierarchical or graph database on the workloads they are designed for. At best a relational database can be equally fast. For this reason hierarchical/graph databases never went away and are still appropriate for specialized tasks. The selling point of relational database is they can accommodate long-lived multi-purpose databases used by multiple different users and applications over time and where the data model would change and evolve over time as the world changes. The is achieved by "data independence" - the logical model is decoupled from physical storage structure and is not hard-coded to accommodate particular query patterns. So you can have ad-hoc queries - cross-cutting queries which you didn't know you would need when the schema was designed. The key insight is that relationships between entities are also just data.