The sad reality is that having something that works, even if badly, is better than having a theoretically elegant architecture that is not implemented. See JS or Linux vs Hurd for other examples.

Ask yourself this question, supposedly somebody made the full implementation of the relationship model into a database engine tomorrow, will you use it yourself, and can you convince your company to use it in place of SQL? Again, I wish this wasn't the case, but I'm not sure if there is anything we can do about the adoption problem.