It is possible to co-opt existing structures, but how do we counter outside forces, like happened during the Spanish Revolution. I could be wrong, but Horizontalism will always be weakened against organized opposition.
That's a completely different discussion. I personally don't think it's as easy to declare that such structures are weaker. The Spanish proved that by fighting for 3 years against overwhelming odds. I think the time that revolution happened was really unfortunate, but the same sort of revolution happening at the time where the surrounding nation states are weakened enough to not be able to intervene might have a vastly different result.
But at the end of the day, the destabilization works, so we keep doing it because those orgs are also the most ethical. What happens if and when capitalism collapses is a bridge that every such society crossed when it reached it, and nobody managed to pre-plan how it would go.