Ummmm..... you are confusing "qualifying for Medicare" with "qualifying for premium free Medicare Part A." You do NOT have to work 30+ quarters to qualify for Medicare, but if you have worked less than 30 quarters, you still have to pay over $500/mo JUST for Part A.If you don't qualify for Medicare you can stay on an ACA policy and probably still receive a subsidy. However, in my state you would only have a choice of narrow network HMOs on the ACA. Paying for Medicare might be a better option.
Also, someone who doesn't qualify for Premium free Medicare Part A due to work history likely wasn't on an ACA marketplace plan, but was on ACA Medicaid Expansion, and they would NOT be eligible for sign up for an ACA marketplace plan due to age nor meet the income requirements to.
And of course, they would not meet Senior Medicaid requirements in most states due to very low income and asset limits. This has come up in several of these threads. Those seniors are getting a very raw deal compared to those through age 64 with the same resources, who get a free ride on ACA Medicaid expansion.