If the Supreme Court throws out only the individual mandate but leaves the rest of the Affordable Care Act intact, it would be a real boon for at least one group of individuals: health care policy researchers.
If we can leave aside the huge moral and legal implications of such a ruling, from a purely research perspective it would accidentally create one of the largest health care policy experiments ever, complete with control groups, one that tests multiple variables.
How this would happen is that the most likely electoral outcome this November is President Obama wins re-election but the Republicans maintain control of at least one branch of Congress. Under this scenario, Obama will obviously not let the GOP repeal the rest of ObamaCare, and it is very unlikely the Republicans will approve a slightly modified mandate or other replacement. Even though the federal government will be unlikely to act, the states can, and some probably will.
It is safe to assume some Republican-controlled conservative states will likely do nothing at first about the ACA lack of a mandate. They will end up serving as a control group to show how effective the reforms, regulations and subsidies are, absent any mandate.
On the other hand, at the very least one state, Massachusetts, is likely to keep its state level individual mandate. In addition, a few other heavily Democratic states where the party is closely allied with Obama and/or the health insurance industry, such as Connecticut, may also adopt a state level mandate. These states will serve as the test cases for how much more effective the individual mandate is at encouraging enrollment, compared to just providing subsidies.
In addition, I suspect that many Democratic and purple states will want to avoid the politically toxic individual mandate, but still want to find a way to encourage enrollment in federally subsidized insurance. It is likely different states will adopt a variety of different alternatives; widely spaced open enrollment periods, back premium penalties, aggressive education campaigns, encouragements through state tax credits, auto-enrollments, default public plans, etc… For example, as Vermont transitions to a single-payer-like program in 2017, from 2014 to 2016 it will probably have something that basically enrolls the uninsured by default in a public plan. Each of these states would serve as a test case for its particular alternative. We will see if these alternatives are more, less or equally as effective as a mandate.
There are currently lots of predictions about what dropping only the mandate would do and how effective other alternatives to the mandate would be, but they are only educated guesses. Most are based primarily on just the model created by Jonathan Gruber, an aggressive individual mandate supporter.
It is possible to point to different models or counter examples, such as the employer provided system or Medicare Part B, to show subsidies and open enrollment alone can produce very high volunteer enrollment, but it is all based on trying to reach conclusions based on imperfectly related examples. There has never been a massive scientific experiment with control groups to truly test these conclusions under these specific conditions. If the Supreme Court strikes down only the mandate, though, we should get one of the largest experiments imaginable.