The same way pensions moved from the defined benefit model to the defined contribution/401k model.
If you have more than 50 full time employees you have to offer coverage or pay a penalty. The penalty of not offering is $2,000 on every full time employee. The penalty is also not a tax deduction. Offering the plan to only your key people is worse than offering nothing to anyone. Unless you're offering the plan to 95% of your full time folks you'll owe the no-offer $2,000 per person penalty in addition to what you're paying on the plan for the key employees. (Not to mention the non-discrimination headaches that would be triggered.)
I don't know what you mean by more people will be enrolled to fund the ACA. Plans are currently being subjected to three distinct fees. The comparative effectiveness research fee, the health insurer assessment (fully insured plans) and the temporary reinsurance program. It's my understanding that the assessment and temporary reinsurance program go directly to the individual pools and exchanges.
The road of least resistance is for an employer is to offer a rich plan and a lower end plan (maybe more options) and price them such that the execs and key people are the ones that can afford the rich plan and the lower plan doesn't cost more than 9.5% of your lowest paid employee's income.
This is why the defined contribution model is looking attractive at the moment. Offer the plans, offer the contribution your employees choose what the want to spend it on.
The hours thing is absolutely happening to employees on the cusp of 30 hours. Those guys will be scheduled for no more than 27 hours going forward.