To clarify, yes. After 2020, the federal government continues to cover 90% of the cost (compared with 60% normally) of the expansion indefinitely. So, for a state government, it is likely the cheapest way to ensure a large number of low income people are insured.
60% is not the normal federal share; the normal federal share varies by State, depends on state economic performance relative to other states, and is a minimum of 50% for certain populations, 65% for other populations.
What you said doesn't contradict the 60%. 60% is the normal federal share. I had a job that specifically involved forecasting state budgets (which obviously involved looking at historical trends) and forecasting the ACA's impact.
There's a difference between 60% being the normal federal share, and a variable amount that, averaged across all states, averages to about 60%, being the normal federal share, particularly when addressing particular states policies, since the share for any particular state tends to be pretty consistent over time (e.g., the California regular FMAP has been the minimum, 50%, because of the states better-than-average economic health for long enough that I frequently have to remind who work on systems where it matters that that's not a fixed amount) and maybe be significantly different than the average.
