First, the direct consequences.
A) The markets actually destabilize. The cost-sharing subsidies are there for the insurance companies who get costly customer bases to be able to afford to stay in the markets. Markets can destabilize in one of two ways. Without the cost-sharing subsidies, the costs could just be passed on to a subset of customers, or the companies themselves could just pull out of the markets. If the customers can't afford the increased costs, the markets destabilize. The latter would probably be even more dangerous. We just got through a period in which several counties, for a period of time, had zero insurers. Then, companies came into those counties to fill the gaps. Without cost-sharing subsidies, though, who knows what could happen? There could be widespread gaps. Anywhere it looks like the lack of cost-sharing subsidies could kill profitability, there might be zero insurers, and everything destabilizes. Or, the costs could just be passed on to customers, and everything destabilizes. Likelihood? Hard to say directly, but high. This is, after all, the whole point of what Trump is doing. This is batshit fucking crazy beyond belief. I have been scouring my political memories for examples of presidents actively trying to cause damage to America as a bargaining chip, and... I can't think of anything. This is just Donald J. Fucking Trump. Think of all of the despicable things that presidents have done in US history. Think of slavery. Advocates of slavery believed that slavery was good and right. As evil as slavery was, its supporters thought that it was good and right. They were seriously fucked in the head, but at least they actually believed they were doing good. Trump doesn't even believe that this is good policy. That's what is so fundamentally fucked here. He is trying to cause damage here. I don't actually know of any precedent here. I used to teach about ideological differences, and about how liberals and conservatives just had fundamentally different world views and economic models that produced different policies, and that if you accept a different premise, you get a different policy, but that each policy is simply motived by a different premise. This really is, for the first time that I can think of, a policy motived by an active desire to do harm to America. I... really don't know of a precedent here. What the fuck?! Anyway, likelihood? Hard to say, but high.
B) The markets limp along. The costs could just get passed on to customers who pay, and somebody gets fucked, but the markets continue to function in some form. Likelihood? By mathematical necessity, low. 1-high=low. That's really all we can say here. As is so often the case with a batshit fucking crazy idiot child of a President, we are in uncharted territory here.
A and B together, though, mean that the economic consequences of Trump's order are negative. That is by mathematical necessity. This is intrinsically bad policy. This came about because a bunch of fucking trolls who were looking for ways to use the courts to undermine Obamacare because they didn't have the votes to do so in Congress found one of the places where the idiots who wrote the law didn't proofread it. The cost-sharing subsidies section didn't do the proper thing in terms of appropriations, and blah, blah, fucking blah. If you care about functioning policy, you do what Congress did back when Congress cared about functioning policy-- amend the law to make it function by correcting the language. Or, you can do what a legal fucking troll would do, and try to sue to follow a strict-letter-of-the-law interpretation in order to destabilize the markets, and use that destabilization as an excuse to repeal the whole fucking law. And if that doesn't work, you can elect a troll as president, and have him destabilize the markets by executive order in a temper tantrum.
Anyway, that's what he is doing. How much destabilization will we see? Who knows? We'll see.
Now, the political consequences.
1) Democrats agree to Republican terms. Trump has actually said that by blowing up the markets, he will get Democratic votes for his demands on Obamcare. Trump is the dumbest motherfucker in the history of politics. That, as a brief aside, is a technical, political science term. Either A happens, or B happens. If A happens, then Democrats don't vote for one of the GOP's failed bills. The healthcare system goes into fits, and the Democrats campaign in 2018 and 2020 against Trump's sabotage of the individual marketplace. If B happens, then it isn't an imminent crisis anyway. What is Trump thinking here? Trump doesn't think. He just does stupid shit. Probability? Absolute mathematical zero. Trump is beyond stupid.
2) Congressional Republicans work with congressional Democrats to fix the loophole that let Trump do this stupid shit. This all comes from the fact that Democrats didn't proofread their own fucking bill. That can be fixed. If this starts looking bad for Republicans because Trump drags down the party by collapsing the markets (scenario A), congressional Republicans could decide (just barely enough of them, anyway) to work with Democrats to amend Obamacare and take away Trump's authority to do this. Congressional Republicans have already shown a willingness to take away Trump's authority on international matters because they don't trust that fucking Russian stooge... Likelihood? Low. This would require a bill to fix Obamacare passing the House, and the Freedom Caucus won't let Paul Ryan do that unless things get really bad, and even then...
3) Senate Republicans get so spooked by whatever is happening that they actually pass a "repeal-and-replace" bill because whatever is in the bill looks better than the havoc Trump is creating. Skinny repeal, Graham-Cassidy... this is all crazy shit, poorly thought out, poorly analyzed, and it spooked just enough Republicans that nothing could pass the Senate. Maybe Trump's temper tantrum spooks enough of them that they decide that one of these hare-brained schemes is at least better than the stupid shit Trump is doing. This is the "I'm going to hold my breath until my face turns blue" method of getting Congress to act because Trump is an idiot fucking child. And it might work. Likelihood? Higher than either 1 or 2, but... I don't see either Collins or Murkowski going along with this. Who else, then, gives in? McCain? Rand Paul? Um... A collapsed market probably sounds good to Rand Paul, who uses that as an excuse for pushing total repeal, which can't happen anyway under reconciliation rules, and McCain has apparently decided that he has had it with Trump's shit, and his shitty diapers. Maybe 3 happens, but Collins and Murkowski aren't giving in, so I don't know who else in the Senate caves.
4) Something I can't imagine. We are in batshit crazy territory here, so I'm just going to throw in scenario 4 as: crazy shit happens because President Camacho is tripping balls on something. Likelihood? Who the fuck knows at this point?
5) Nothing. Everything just limps along because the GOP is a totally dysfunctional party. Likelihood? If I can't estimate the likelihood of 4, I can't estimate the likelihood of 5. Sorry. We are completely through the looking glass here.