Yes, 1st level characters have always been vulnerable. But not this vulnerable. The power of 5e 1st level characters is higher than B/X, but kobolds in 5e are Much more powerful than their B/X counterparts. Same thing for orcs and hobgoblins and ogres.
Only 4E creates a game where 1st level characters start as super heroes. Every other edition, you are dealing with above average characters. They are skilled but still have to play smart and use their heads to succeed. This creates a better game where tactical thinking, and strategy plays an important role. Your players can't just rely on the rules and the rolls of the dice to win... they have to think and plan tactically.
It’s vastly more “realistic” that every monster should have its own initiative score. Kobolds are not telepathic, they don’t have a hive mind, they are each individuals who should act separately. There is no reason for them to be more coordinated than the PCs. Realistically they should each be played by a different player, that have individual values including self preservation. But that’s hard to run, so the compromise is the DM runs them all.
Group initiative exists as a game concept, which makes things more game-like. The DM often has a lot of stuff to do and keep track of, and hence all monsters that work the same, all go at the same time, to make things easier on the DM. Splitting each monster into an individual initiative is more realistic, but makes more work for the DM when dealing with multiple monsters. Luckily at low level, the DM doesn't have that many monsters to track in a given fight, so it's not necessary.
Having all the kobolds work in perfect unison together is unrealistic, even fantastical (which is OK because D&D is a Heroic Fantasy). Beyond ease of use for the DM, group initiative can make low level creatures dangerous to higher level groups. It’s not done for the sake of realism; it’s done to decrease complexity and increase challenge. Group initiative is an unrealistic tactical advantage for creatures which already have Pact Tactics.
At level 1, in 5e, that extra challenge isn’t necessary. You can run them that way, if your players really enjoy a challenge, or if your explicitly running a meat grinder style game. But that's a stylistic choice, not an attempt at "realism."
A lot of interesting and true things have been said already. Little to add to, except...
Knowing this, the danger of even low-level monsters such as kobolds and goblins, I started to think about my encounters a little more. You don't need to have a warband of 25 kobolds in the forest - each and every - time. Have a patrol of three or four show up. Have one or two show up, and make the encounter where they start to run away to warn that group of 25. A chase after one or two of the little buggers can be quite a refreshing different take on the kobold encounter, raising the stakes quite high (you don't want theother kobolds of the warband after you inone big, fell swoop).
And with AC scores not soaring into the 40's and 50's like in previous editions, low-level mooks can stay a (minor) threat. Certainly that kobold with his impuny weapon only deals that D4 with a minor bonus here and there. But they are still able to his most characters, and then having this group of mid-tier characters surrounded in a sea of kobolds using pack tactics, all those D4s accumulating, softening up the party for that monster that will be a greater threat...