That wouldn't make the ACA collapse, it would just make premiums more expensive in those areas for the government. Seriously, just go read nonpartisan analysis of the ACA like the CBO. None of them think it is in danger of collapse. This is a right wing fever dream. Regardless if the ACA IS going to collapse the republicans run the entire government. They will be blamed, and rightly so. If they genuinely think it is going to collapse and are choosing to allow millions of people to suffer for an imagined political advantage they all deserve to be voted out of office.
It says a lot to me though how half-assed their 'repeal' was, which to me says they don't believe their own rhetoric that they're selling you. Everything in their repeal bill would make adverse selection WORSE, not better. All that bill was was a rich person tax cut disguised as a health care bill. They were playing you.