The fact that a system requiring ad-hoc attributes wouldn't work so "hot" in a relation isn't a problem with the relational model. This difficulty comes from the application of the relational model bringing to light that such a situation is necessarily more complex than one without ad-hoc attributes, and you cannot treat such data the same way as you can treat data in a relation. The important lesson is that you cannot abstract this added complexity away without losing something else of value (data integrity and simplicity of reasoning).

This gives you more power when your situation isn't ad-hoc attributes (by providing more guarantees) and lets you model the ad-hoc situation with a EAV model if you must. SQL maybe should provide some more sugar for accessing that EAV, but it's not a fundamental relational model issue.