Well, actually, you can't buy insurance that is "Obamacare." You can buy a policy from BCBS or Cigna, or the dozens of other health insurers out there, ON and OFF the exchanges, with premiums and plan details set by those companies etc. but there is no policy anywhere called Obamacare, and there is no monopoly.
You know that the GOP took the House in 2010, and spent the next 6 years taking about 87 repeal votes, right?
And I don't really know what your point is here. What Trump is doing is deliberately harming those on the exchanges, trying to increase the premiums they pay, make their lives worse off. They did it first by shortening the sign up period by about half, then by almost eliminating advertising and cutting funds groups that help people get into a good plan. The only end point can be fewer people with insurance, and higher premiums for those who are insured. So because that's the inevitable result, we have to assume that is what Trump WANTS.
And the reason that's the strategy is the GOP and Trump can't say, "repeal the ACA because our plan is BETTER!" No one believes that. Their plan after 8 years of promises we all knew were BS was a plan that as I recall polled about 14% and was opposed by every major group with an interest in the legislation, including the healthcare industry, insurers, their own GOP governors, and conservative groups. So the only option now is to intentionally burn down the ACA, so we can replace it with "Who the hell knows what these lazy morons will come up with?"
If you want to defend that, be my guest. I think it's pathetic, and a perfect illustration of where the Republican party is in 2017. They can't sell their ideas, not even to their own governors, so they'll burn the system down and leave us with no choice.