SpaceX's genius has never been getting the absolute maximum possible thrust out of engines. It's been designing them so they can be mass produced inexpensively, and re-used. They've proven that is the far more important skill.
So making the Falcon Heavy work as simply and well as possible is the important part. Getting another 10% by making crossfeed work, at the cost of making it more complex, harder to build and re-use, simply wouldn't be worth it.
