According to Jim Cavanaugh, the bomb maker involved in the Barcelona attacks probably blew himself up before the three vans were filled with explosives.

Possibly, but, there were others involved in the attacks and vans could be used to carry more into the scene. I think the exact attack needs to be understood, but, the absent bomb maker is a viable scenario. 

The suicide belts were absent of explosives, too. The presence of the suicide belts also provides an advantage in the view by law enforcement. If the belt is hit by bullets they would possibly explode. There is a psychological advantage to those dynamics. 

If this was a botched attack it was still effective because of the delicate nature of the human body sitting on sidewalk cafes.

Barriers on ANY sidewalk cafe should be an international directive. The barriers can be decorative and still be effective. Artists will have ideas.

Barriers improve the survival rate of people targeted in any such attack. Lesser countries trying to build their economies don't put these measures into their plans. They should.