> Healthcare costs [...] The only difference is with an employer intermediary, [...]

That's missing the biggest problem, which is that the employer gets a free chance to extort the employee in all sorts of illegal ways lest they be cut off and die.

Wage theft is perhaps the biggest-value type of crime every year (sources disagree, but it's certainly higher than many), and that's only one kind of illegal thing employers do when they have all the leverage.