I was under the impression that part of the reasoning for the potential for concentration to break on damage was to limit the power of gishes and ensure that the couldn't overshadow the martial classes in melee. Yes, if you're trying to optimise everything by game mechanics, then having combat spells using concentration is suboptimal. That's the point: you're taking a risk where a melee Fighter or Barbarian for example with less flexibility is much more reliable.
I'd rate a subclass ability that let a gish bypass the concentration mechanic as very powerful when considering balance. Consider the availability and mechanics of the Warcaster feat and suchlike when designing that ability as well.
You might be reading more into it than I intended.
I'm not talking about complete freedom from Concentration as if that's a considered mechanic I'm actually proposing - I'm just talking in general concepts here.
That damage breaks concentration is mainly a general threat to every spellcaster - no longer do you outright need to kill them to get rid of a pesky spell.
The notion that you'd willingly go into melee buffed by a Concentration spell is deeply flawed, however. It's just not worth the risk. These spells show no signs of being sufficiently buffed to entire players to use them that way. And there are always alternatives - if the game truly supported the idea of melee Concentration spells, it shouldn't have provided other spells that are equally desirable, yet does not require Concentration, or work only in melee.
Take Hunter's Mark for instance. For a ranged Ranger, a neat spell. (It's probably not as great as everybody thinks it is, since it eats up your bonus action too often, and an optimized build already uses that to deal damage)
But for a melee Ranger, an outright trap. Neither spell nor class provides any extra benefit for using the spell in melee, so it's plain worse than using it at range (where you can expect to take significantly less damage).
So, no. I don't see any sign that the breaks on damage rule of Concentration has much to do with gishes at all. Unfortunately, the simplest explanation is that this case was simply not playtested much, if at all. It probably was never a consideration.
Which is why the subject of messing with the rule came up.
Which is why I'm reminding everybody you don't necessarily need to mess with the general rule - you could instead just give gish builds special abilities that allow them to use melee Concentration spells with less drawbacks than back-row Wizards.
