Make/break arcs are what destroys non-HP-rated relays or switches
with motors.

Secondly, due to erosion, the contact area (and hence resistance)
will vary over time. A tiny difference in absolute terms will make
a _huge_ difference in current balance.

I'll buy that. It will arc on a single pole first, and last. In that
case,
the resistance *decreases* with the current (that is, zero current =
no ionization = high resistance). Then I don't know what happens.
If erosion increases the distance between the contacts, then the
closing arc will favor the less eroded contacts. But the opening
arcs?

Trying to reason this out is just too messy. Thank you for the
clear analysis. You don't happen to live near the San Francisco
Bay Area, do you?