The mandate, though is an essential part of the scheme because without it you would also have to allow denial of coverage for pre-existing conditions. The two go hand in hand.
If you get rid of the mandate but continue to require insurers to cover new sick people, there is no longer any reason for anyone in the country to buy “insurance” until they get sick.
And that's why Obama seems confident the insurance will take hold. Assume someone gets sick or has an accident or something that would've been expensive to pay for. So the patient is told "it's covered" and they only have to pay the deductible if they sign up. So they sign up and it'll be as an automatic deduction from a bank account or a charge to a credit card. You think that person will ever in their lives get to opt out of those automatic payments?
That's how it worked in Maine...."Guaranteed Issue"
It totally destroyed the actuaries' ability to assess risk. The IM rates were the second highest in the US.