It wasn't a small part of it.
For insurance companies to be able to afford the requirements placed upon them by the ACA, people need to be buying insurance all along, or they need to pay the penalty for not buying insurance all along but being allowed to jump into relatively inexpensive coverage months after they get an expensive diagnosis.
That's only an argument for eliminating guaranteed issue and community rating if the mandate goes (i.e., bringing back pre-existing conditions). It's not an argument for striking down the entire Affordable Care Act, which is substantially larger than those particular reforms, and substantially larger than insurance market reform in general.
The ACA revamped the entire philosophy of the Medicaid program, turning it into the program most people probably thought it already was. That doesn't have anything to do with the individual mandate.
It's been driving redesign of care delivery in this country to make our delivery system more efficient, patient-centered, and high quality. Hundreds of health care systems around the country treating millions of patients now rely on business models that rely on keeping patients healthy and targeting care to them more effectively. That doesn't have anything to do with the individual mandate.
It makes huge investments in the nation's public health infrastructure, everything from the clinics in our schools to the management of chronic diseases in our communities. That doesn't have anything to do with the individual mandate.
It made huge investments in building up our health care work force, paying to educate, train, and retain thousands of new providers--doctors, dentists, pediatricians, community health workers, mid-level practitioners, you name it--and building new community health centers. That doesn't have anything to do with the individual mandate.
It eased the FDA pathway for new biosimilars to get them to market faster, created PCORI to build and disseminate the evidence underpinning treatment decisions, it launched a concerted effort to improve patient safety in the nation's hospitals, which has been a serious cause for concern for decades. None of that has anything to do with the individual mandate.
All of that, and a thousand other things improving our health care system but not logically or economically connected to the individual mandate, would be gone if this bizarre and foolish ruling stood. It won't, but let's not pretend there's some argument that most of the ACA has anything to do with the individual mandate or can't stand in its absence. Even the idiots in the Trump administration were only arguing that pre-existing condition protections need to get thrown out if the individual mandate falls. Not the thousand other reforms in the ACA.