Let's be honest here. The GOP efforts are not really about healthcare at all. When the ACA was created, the "deny Obama any victories at any cost" strategy was in full swing. They tried a series of power plays to kill it, and they lost. And ever since then, they have been trying to reverse that loss. 
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The problem is that many provisions of that law have become immensely popular, enough to make it impractical to eliminate the law entirely. They are stuck with a balancing act, trying to forge a "moral victory" that would let them say they "repealed Obamacare" without enraging their own supporters and the unaffiliated voters. To do that, they have gone from the rhetoric of repeal to one of "repeal and replace" with all kinds of wonderful predictions and promises for the benefits that replacement will bring. 
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They could have changed the problematic parts of the law at any time. But that would not give them their retroactive victory. They still lost.