I found your example to be contrived in showing absurdly low damage to try to emotionally support your point. Fireballs can't go that low, and if they are fighting foes that are throwing fireballs then a melee attack is unlikely to only do 2 damage even with rolling minimum. The exact same thing could have been:
The wizard fails a save vs. Call Lightning for 18, gets critted by an Ogre for 20, and makes a save vs. Fireball and also takes 20.
It's the same three DC 10 saves. But it sounds a lot different.
Though you're right on the latter part, please not the OP didn't mention any fireball. Firebolt is a cantrip, and even if 2 is a bad roll, it's possible.
I think his goal was to show that rolling at each damage, even very low, makes it virtually impossible to keep concentration if a party is focusing fire on the BBEG, even if they don't do enough damage to drop him. (I found this situation to be more common than the reverse, but maybe I'm going soft on my players).