I'm not sure what folks expect Trump to do. You've got the Freedom Caucus that won't vote for anything less than (close to) full repeal, including repeal of the 26 year old addendum and guaranteed issue. Then there's another group of moderates who won't vote for anything that doesn't retain both. And each group is insisting that it won't settle for anything less. The Senate is even worse.
Regardless of what Trump says, a bill isn't going to make it to his desk unless enough members of Congress will compromise, and not enough are willing to do that yet. So, Obamacare will stay in place. The exchanges will eventually crumble, although the 26 year old provision will remain in effect, along with guaranteed issue and the Medicaid expansion. Trump isn't as much making a threat as just stating reality. And if there is no willingness to compromise within Congress itself, he might as well move on to other things that have a chance of passing.
As goofy as he is in a lot of respects, he's handling this like an experienced negotiator. If there is no middle ground that is acceptable to both sides, then you're wasting your time negotiating. If it's litigation, that's when you go to trial. If it's a business deal, that's when you stand up, walk away from the table, and move on to something else. Maybe one side or the other will soften its position over time, but if not, it's a dead issue.
Obamacare, with all its flaws, will remain. Not a threat -- just reality.