The first deadline for insurers to commit to participating in the Obamacare markets was this week in Kentucky and Virginia, and they're still in:
Initial filings from Virginia and Kentucky show that insurers are still contemplating offering Obamacare coverage in 2018, even amid continuing uncertainty over Republicans' plans for health reform. Nine insurers filed to offer plans in Virginia, one fewer than this year. Two filed in Kentucky, also one fewer. […]
Virginia and Kentucky are among the states with the earliest filing deadlines, and insurers won't have to submit their premiums for 2018 until later this spring. The Department of Health & Human Services pushed back the federal deadline until June 21, and many states followed suit.
Also, filing now doesn't mean insurers are definitely participating next year—they have until September to withdraw. Several carriers have said they are still contemplating their involvement in Obamacare for 2018.
Sticking it out would be one way to stick it to popular vote loser Donald Trump, who continues to threaten them. If they don't play his sabotage game, there's less a chance that he gets his desired outcome—Obamacare collapsing without his intervention.
Congressional Republicans are quickly coming around to the realization that a replacement plan isn't viable, and that if they do real damage to the law that results in real people being hurt, they're going to get the blame for it. This gives power to the insurance companies. If they stay in the game, they up the chances of the law's survival and justify the last several years worth of investment they've made into it.