I also suspect that the battery–particularly if it is the original–on this 8 year old vehicle is the actual source of the problem. Even if the battery is not the original one, by my standards this car should now be on its 2nd replacement battery. The battery and the alternator work synergistically, and a weakness in one will inevitably lead to the failure of the other.
A possible scenario is as follows:
The original failing alternator may have led to a weakening of the battery.
Then, this weakened battery needed to be charged so much that it quickly overheated and killed the subsequent alternators.
It is difficult to determine the exact cause and effect here, but since a battery is much cheaper than an alternator, and since I do not like to be stranded by a battery that fails without warning, I tend to replace my car battery every 4 years or so. I consider this to be preventive maintenance, even though it is not listed as such by any car manufacturer.