The designers or even third parties could slap concentration on every spell that comes forward and it will not do much to balance a spell (except to limit use at any given time).
Well, no, not really.
My point was that by "forgetting" to add Concentration, you can break not just new spells,
but existing ones too. Any given Concentration spell, let's say Haste for sake of argument, could well be overpowering right now and still not breaking the game since everything else you'd want to combine with Haste requires Concentration too.
A spell could arrive that, by itself doesn't look too bad, even with Conc left off. But if this spell - continuing our example - acts as a "haste multiplier", that spell could well break Haste.
Now, back to your post.
My best response is to view Concentration as a necessary but not sufficient parameter for
overall spell balance in this regard.
https://en.wikipedia.org/wiki/Necessity_and_sufficiency
That is, you are right Concentration in itself doesn't guarantee or even impact
individual spell balance. For many spells, the presence or absence of Concentration does not matter one bit.
But that's beside the point - being
the lack of Concentration could well wreck the balance of completely unrelated spells (which
could have Concentration).
TL;DR: Don't think of Concentration as a ward against imbalance. Do think twice before adding a spell that does
not have Concentration, especially if it buffs or debuffs creatures for a duration.
