What? Yes, this is a good thing.
It is commendable that the politicians in Arizona wanted to protect the people when the Federal government failed to do so.
But it's not theirs to do, nor more than could Massachusetts decided to levy a tax on imported medical devices to protect its medical manufacturing base. Some things are just exclusively the purview of the States and some things are Federal jurisdiction.
The reinforcement of Federalism goes towards putting the roles of State and Federal governments back in their respective places, and indicates we should be optimistic about the future of the 10th Amendment balance between Federal and State governments.
What if Aizona had won? The Feds could no longer be held exclusively responsible because they had ceded key parts of their jurisdiction to the States.
We have a Federal government that has badly overstepped its bounds and run roughshod over the states. The remedy for this is not to apply the same malfeasance in the opposite direction, but to robustly define the bright line between Federal and State roles. In short, we need to reinvigorate Federalism.
If the Court had allowed this Arizona law to stand, it would only add to the confusion of fed vs state law and create even more chaos.
While it is desireable that the laws be enforced, the States taking matters into their own hands only increases the lawlessness.