So if you set up a corner case scenario where a fireball can take out all of the enemy, which are all spellcasters, counterspell doesn't work well. Well, you're right. In that corner case it doesn't. In a normal combat where there are 1 or 2 enemy casters, you are better off stopping what they are doing most of the time. Especially in 5e where things have more hit points, so a fireball is not likely to put the enemy down like that. Actually, in your corner case the party gets destroyed by only 5 fireballs, not 6. The 6th caster is going to be counterspelling your fireball.
Your logic with the extra restrictions is also mortally flawed. Some restrictions =/= the stifling restrictions of 3e. There is a middle ground.
I was pointing out the opposite corner case and my assumption was that the enemy didn't have Counterspell available. It is not that the logic is mortally flawed, it is that counterspell is somehow a great spell that needs to encumbered with more restrictions on it than already exist. As I pointed out, having multiple party members with counterspell is a corner case. A mild perusal of class popularity supports that supposition.
There has been really zero justification in this thread for adding more restrictions to a spell that has very limited utility to begin with. There are only a few very extreme corner cases in which one could claim there is a "problem" -- a problem that is easily solved by encounter design if necessary.
Change for Change sake isn't "middle ground".
Based upon RAW, you have no idea as to what spell you are countering since it takes your reaction to identify a spell as it is being cast in the first place. You also have no idea as to the level of the spell being cast either (ok if you don't have to make a roll you know it was your spell level or lower). So you are going to gamble with a 3rd level spell slot (or higher) to shut down a spell that may or may not be worth it? Congrats, you just countered a spell. But you also don't know what spell you have countered either. So you could have counter a
fireball,
conjure animals or just a
fire bolt.
The opposing caster could just keep casting
light over and over again to force your
counterspell happy party to waste their spell slots. There are 42 "official" cantrips, 72 1st level spells, 71 2nd level spells, and 62 3rd level spells that can be countered without a roll. If you can figure out the type of spellcaster you can narrow that list down somewhat, but still you literally have no way of knowing what spell is being cast / countered.
Round 1
DM: An enemy starts to cast a spell
Player Reaction: I counterspell it
Player: Cast Spell
Enemy Reaction: Counters it
Round 2
DM: An enemy starts to cast a spell
Player Reaction: I counterspell it
Player: Cast Spell
Enemy Reaction: Counters it
Round 3
DM: An enemy starts to cast a spell
Player: I am out of counters
DM: Boom, fireball everyone make a save.
All that has happened is that the two spell casters are out of the fight until they run out of spell slots. So if the party is facing two spell casters would counterspell be useful? Player only as one reaction per turn and still has no idea what spell was cast or countered. A smart player who faces enemies who can counter him has plenty of counter measures to prevent the use of the spell (you do have ranks in Stealth I hope, at the very minimum.)
All of the hand wringing over the supposed power of the spell is just white room concerns. In the action economy, the spell just isn't that powerful.
Counterspell adds an interesting poker / bluff component to the game and it might actually get used once in a while compared to previous editions. That is about it.