Greetings,
I'd certainly agree that endgame knowledge is very important to a player's development - without it, most players will either try to avoid endgames or fail to win/lose them because they don't know what to do.
Having said that, a certain knowledge of openings - and traps to avoid! - is necessary. The
Tactics in the Chess Openings series is a particularly useful guide, in conjunction with Collins'
Understanding the Openings. The former will certainly appeal to most developing players as it involves the one thing they like - tactics!
Along with endgame knowledge, and perhaps more important, is the ability to analyze - again, this includes tactical vision (pattern-recognition) as well as the ability to calculate.
With these three - tactics, endgame and opening knowledge - most developing players could "get by".
If they want to increase their opening knowledge, they could add suitable specialist books on their specific repertoire.
At a certain level, they'll then need to develop a greater awareness of positional play - which will take them up to (and beyond) the 2000 mark.
I also agree with
ErictheRed's comments about games being lost due to errors other than in the opening.
As a teenager, I once played a game in a tournament where I dropped a piece through playing the opening on "auto-pilot", but then outplayed my opponent through superior tactical ability and endgame knowledge.
The latter came into play both in the late middle-game and the actual ending.
In the middlegame, I was concious of which pawn formations would be advantageous in an ending.
In the transition to the endgame, I showed my middle-aged(!) opponent how to create a passed pawn from a three-against-three pawn formation (central pawn advances and no matter which way the opponent captures, a passed pawn is created...but you know this already!) - my point, though, is that this isn't normally found in books on openings or the middlegame(!)
There were some other endgame "tricks of the trade" which I was able to use as well - my opponent was suitably impressed, makings "Ahs!" of delight - on my way to queening the passed pawn ... before blundering her to a knight fork with check - the game ended in a draw.
(The blunder was due to having had to concentrate for so much of the game - until the pawn queened, when I subconciously relaxed.

Having queened, I even checked and double-checked that the knight check wasn't possible - it seemed that there was a phantom file between the king/queen and the square on which the knight would land! Mental tiredness...

)
Still, the point only goes to show that if I hadn't had the endgame knowledge I did at the time, I wouldn't have been able to gain a winning advantage on opening/middlegame tactics alone.
Lastly, talking of "tactics" and "endgames" - has anyone read van Perlo's recent book,
Endgame Tactics by any chance?
It looked/sounded rather interesting.
If it is a good book, might it be suitable for the afore-mentioned teaching of endgames, given that youngsters would more readily accept endgames with "tactics" than the rather dry material upon which most endgames are based?
Kindest regards,
James