I think it's a good thing that concentration can be broken with damage. The concern I have is that the DC goes up the more damage a caster takes. Damage scales considerably as you go up in level, while things like checks and saving throws are subject to bounded accuracy (hit points and damage are not bounded). At higher levels, the DC is going to often be much, much higher than 10, while the caster's con saves haven't really improved at all (since neither clerics nor wizards are proficient with con saves). If a caster is struck by a higher level effect that does 40 or more points of damage (which I imagine is fairly common at higher levels), he's pretty much guaranteed to fail the saving throw and lose his concentration.