Flexible Database as a Service

I was wondering if the following idea is worth pursuing:

A Database as a Service where users define the datamodel as triples (Just like in a graph database).

If certain parts of the datamodel get queried often together there is a possibility to migrate only those parts to a more traditional database (document storage/sql) with a mouseclick, thereby trading in flexibility with performance.

The advantage is that most database designers are not able to define fixed schemas when starting off a new project/startup, resulting in quiet an overhead to decide which "tables" and attributes to create. By giving them the option to define only triples this task is put on hold until actual queries show which attributes should be indexed "together"

Looking forward to an interesting discussion