What will happen to Obamacare if...?

That's not quite how it works. 0bamacare is not a plan, it is a regulation. You will be required to have health insurance. You can get it either through your job, the state, the new "marketplace exchanges", or as an individual policy. No matter which way you get it you are getting the coverage through private insurance companies and in addition you are "funding" the regulation.

The reason they state that young adults are going to "fund it" is because young adults usually do not use too many medical services. Also, the spread of premium between younger people and older people will shrink. Younger people will see a much higher increase in premium no matter how they get the policy. So in essence you are subsidizing older people.

As an example, in my state right now someone in their 20's can get a good policy for around $80 per month and someone age 60 would pay around $750 per month. After 0bamacare the 20 year old's will be paying around $300 per month and the 60 year old's will be paying around $800 per month. However, the premium is not the end of the story. Right now many people age 60 cannot get insurance because of pre-existing conditions. After 0bamacare they can, meaning there will be many more 60 year old people with major health conditions on the plans. If the 20 year old's didn't purchase any insurance the insurance companies would go out of business.