Employment ain't guaranteed either, so I'm not sure that guarantee is worth much.

And you can be pretty sure that many workers would walk if the extra goodies they are accustomed to but which ain't guaranteed were to be withdrawn. Assuming the competition still provides them.

Employer contributions to SS taxes are guaranteed by employment. Also other payroll taxes added on by states.

Why would that make any difference? What an employer spends on an employee is what the employee gets. If the employer spends less, the employee gets less, and hence is a pay decrease, even if not in nominal terms but in real terms.

Premiums for a silver plan can easily be $30k per year for a family of 4. If an employer decides to cover 70% of it instead of 80%, that is literally a pay decrease of $3k, not to mention possible changes in coverage, deductible, and oop max.

For example, the employer could keep the 80% subsidy, but increase deductible from $1k to $10k. Unless premiums go down a lot that is basically a huge pay decrease too.