There are a few ways to interpret that.

If you mean that the companies would stop offering healthcare altogether, then they have pay a tax penalty based on their payroll. Smaller companies might do that. Maybe. But they're likely to either not currently be offering health insurance at all, in which case they're probably small enough that the penalty won't apply to them, or, if they are offering it, their costs are probably pretty high. I fail to see a downside in this situation.

If you mean that the companies would stop coming up with their own plans and instead offer the plans that are available in the government-sponsored marketplace, but still share the costs with the employees, I don't really see how that's different from the company simply finding a new plan, and, as a point of reference, I've worked for my current company for about 2.25 years, I have had three different health plans, and that seems not out of the norm based on prior experience and conversations with other people, so I don't see how that's much different either. The point being, your employer is already likely inclined to change insurance plans every year; what difference does it make if that plan was defined by some HR rep or someone else?