Paul Krugman provides the simple answer to the above question. There is no conservative alternative to the Affordable Care Act. Krugman, like many of us, would have preferred a single payer system like that in Canada. That was not politically possible even when Democrats controlled the House and the Senate. The ACA was modeled after the plan developed by the conservative Heritage Foundation. The basic idea was to increase competition between private insurance providers. That was a market friendly approach that kept the insurance companies from fighting against the plan. However, the plan would only work if certain conditions were imposed. Subsidies had to be provided to those with low incomes, and mandates were required so that healthy individuals would keep the available pool of applicants from being dominated by those with preexisting health issues. The Republican Governor of Massachusetts passed a similar plan in his state with help from the Heritage Foundation. There was no opposition from the Republican Party to Romney's plan.
The Affordable Care Act was a different story. The basic problem is that it was an Obama plan. The GOP was doing everything that it could to delegitimize President Obama. Attacking the ACA was just part of that strategy. The other problem is that it was a federal government plan. Extending health insurance to needy families, many of whom were minorities, was an easy target in red states. Obama hatred in red states is rampant; the ACA has been a favorite whipping boy for red state politicians. Consequently, GOP politicians continue to appease their base by proposing bills to repeal the ACA. They have done so even though they know that it cannot be done without damaging millions of families who have taken advantage of the ACA. Their problem is that there is no workable alternative to the ACA that does not provide subsidies to low income families and use mandates to make the insurable pool satisfactory for insurance companies. They are left with the problem of attacking a plan for which they do not have an alternative. Their only solution would be no plan at all.