I'll have to find out what fretting is before I can comment on that part. However localised heating is only a significant issue if the heat cant get away from the pins, some heating at the contacts it normal and expected. The issue here is that at very high currents the heating is destructive, at low currents it may very well never cause any harm, since the heat moves away fast enough that oxidation never gets a chance. I wouldn't like to guess at lifespan vs voltage though.
Fretting is a mechanical issue, what source of motion are you concerned about? I can't see vibration being an issue as the chip is rather firmly clamped down, so presumably expansion/contraction with temperature as it cycles on/off idle/load? I think I agree with you here, dodgy contact combined with the pads scraping across each other as the surfaces get hot/cold is not a desirable thing. Again, I wouldn't want to estimate the lifespan decrease.
I think, were I to own one of these boards, I'd be really annoyed at foxconn but not justifiably so. It's unlikely to kill anything within warranty period, and there are enough modes of failure for electronics anyway that adding another isn't that big a deal. Just don't run them under phase 24/7. I wonder how many similar faults have been present over the years that we just haven't found out about.