The SCOTUS didn't "rewrite" the ACA, it interpreted the individual mandate penalty as a tax, mainly because the amount of the penalty was far less than the cost of paying for health insurance. The ACA doesn't unlawfully compel a person to purchase a product or service, and this is proven by the fact that thousands if not millions of younger Americans are rationally deciding to pay the tax rather than purchase health insurance they feel they don't need or cannot afford.
It is important to keep in mind that the idea of an individual mandate is to encourage a wide spectrum of folks to join the insurance pool, so that rates can be affordable for those with pre-existing conditions (who, but for guaranteed issue, would be uninsurable). As much as most of us despise the ACA, we should keep firmly in mind that the alternative, in order to solve the access issue, really is compulsion - the requirement that we must all accept government-run health care financing under a single payer system.
Philosophically, the SCOTUS's ruling was "conservative" in two important ways. First, as noted above, the practical alternative to the ACA (or similar system based on an individual mandate or tax) is single payer. Second, for better or for worse, the ACA WAS the product of the deliberations of our elected representatives. I agree with Justice Roberts' conception of the court as deferring to the decisions made by our elected representatives when at all possible. That's how it's supposed to work in a representative democracy. As Luis has pointed out, if we don't like the ACA, we need to elect new Congress-critters who will abolish the law or fix it. It's not the responsibility of unelected philosopher-kings to do our dirty work for us. If we can't change the ACA by means of the democratic process, and instead rely on the SCOTUS as a kind of deus ex machina, then we've sacrificed a lot of what makes our American system of self-government the envy of the world.