In any case, you've missed the point. If Amazon is competitive and better than others on pay and conditions as you say, sectoral bargaining would be good for them, because it would stop them getting undercut by worse employers.
> Firm B would be pretty easy to compete with, because it would end up with the worst human capital out of all of the firms, and a firm with better people would find a way to add value that Firm B could not.
This is obviously not true. Plenty of companies pay terribly and get away with it, and plenty of companies thrive because treating employees badly is their primary innovation. Uber, for instance, has found success through paying "employees" effectively below minimum wage.