> the countries with sectoral unions all compete relatively well.

Relative to what? The question isn't really whether Poland is more or less competitive than California (the other differences between them would dominate), it's where they would each be with the other system.

> And the cohesion provided by sectoral unions is enough of a social benefit that you rarely find their employer class willing to destroy that contract.

It also tends to result in market concentration because a startup who can't hire in an industry without negotiating with a huge existing union is put at a disadvantage relative to large incumbents, and the incumbents may like it that way.