those conclusions are a bit misleading .
in your first example, let’s say the monster has 100 life and 95% resist after sunder and le’s say you deal 100 damage.
without no -%rez gear you would deal 5 damage to the mob each hit, taking you 20 hits to kill it.
after you equip your -% rez gear ( -crescent, - griffon, - merc infinity) and the mob having only 18% it will take you only 2 hits to kill it (82 dmg first hit and 18 dmg the 2nd hit)
so no, your damage does not just double, it will be 10 times larger, even with the monster still having some resistances left.
that’s why -% resists are really overpowered in this game once you sunder the mobs.