At an air show I saw three A-1os in attack foration. They worked in a rising and falling gyre. One attacked, one covered it, and the one one top covered high threats to the ones below. It was amazing to watch.
In Vietnam the job was done, effectively, by Cobras and Phantoms. But even together they couldn’t match the firepower and flexibility of one A-10.
Until the end, at An Loc, the NVA never was able to bring modern AA or SAMs to bear in the south. But you are right, if they had been able to from the start it would have been a game changer, regardless of the attack aircraft involved.