You seem to be assuming that voluntary is more efficient and you seem to be assuming voluntary results in a higher contributions through some kind of adequate participation rate.
Obviously voluntary contributions are more efficient because people resist being forced to do stuff. They evade, avoid, run, hide, and fight back. Monopoly welfare providers also face no competitive pressure to be efficient.
If participation is inadequate, we have two options: I prefer persuasion and you chose coercion. The danger with coercion is that the coercive mechanisms remain in place and can be used for socially harmful as well as socially beneficial ends. Taxes fund war, graft, secret police, etc.