As Squiggy said, that demand is already there and being met at the ER, at the extra expense (via taxes) of all.
I think the impact of abuse of ERs is overstated in this case.
I think the number of people who would go to the doctor if they had the flu if they had insurance is much bigger than the number of people who currently go to the ER for the flu with no insurance.
This isn't just about "abuse" of ERs. This is about people who have no choice but to wait until their medical condition is bad enough to go to the ER instead of having problems taken care of early when they come up. Emergency care is much more expensive, time consuming and resource consuming then ordinary preventative care. And this is a primary casual factor as to why health care in the US is so expensive... everyone who has insurance is already shouldering the burden of the cost of care for the uninsured in the most inefficient way possible.