The best argument against democracy is a five minute conversation with the average voter, as said by someone but who probably wasn't Winston Churchill. Dunno why it'd be any different within a workplace versus politics. (Of course there's also the idea that democracy is the worst except for all the others, which... I suppose. It's certainly better than what the fascists and the tankies come up with although that's not saying much.)

Also, this has certainly been tried although admittedly not in a holistic manner. The Nordic model is basically this blended with more traditional ownership structures. A mixture of the welfare state providing major services with privately owned enterprises alongside co-ops and "democratised workplaces." And... it's fine. Has some nice qualities when compared with contemporary socio-economic structures but the political process can muddy the waters and make things inefficient through perverse incentives. Not that similar perverse incentives don't exist elsewhere, but y'know...