1: Fischer never gave a reason why 1.e4 was best by test.
2: 1.d4 Nf6 is not an opening, and it can lead to the QGD after 1.d4 Nf6 2.c4 e6 3.Nf3 d5.
3: The Exchange Slav is tricky to handle, but would you honestly rather face the Marshall Gambit?
All main openings are equally good in my opinion (QGD, Slav, Grünfeld, Nimzo, Queen's indian, etc, etc...).
What matters is one's ability to play them instead of the opening in itself ;).
It is useful to know the strengths or weaknesses of an opening but it's always very hard to exploit it when your opponent plays normally, otherwise they wouldn't exist anymore!
You can master the QGD or any other system but you will still lose to a better player. Practice middlegame strategy and tactics instead of openings.
@Avorton2b openings is important too, to master them is also important. Rather pick openings which suite your style, as to play better in middlegame and/or endgame
1. d4 Nf6 can actually transpose in many different systems. Who said you have to go 2. c4 e6 3. Nf3 d5? At a bit lower level most players go 2. Bf4 - London System and that is a completely different way of playing. 3... d5 in the aforementioned move order is also replaced by 3... b6 quite often which is QID. That's like calling 1. e4 d5 "not an opening" because nobody has said that you have to take on d5. The list goes on.
I forgot to mention - how about if White answers to QGD with 4. g3 5. Bg2? That may be something to consider when playing d5 e6 and Nf6.
@kiritolemon Of course, all aspects of the game are important, but I think it is useless to devote hours and hours to theory under 1800-2000 FIDE, in my opinion. i would focus on other things in his case
Yeah i havent actually played QGD or Slav. Ive only studied the Semi-Slav and liked it, but liked Grunfeld and Benko much much more
Fischer called 1 d4 "dull and drawish"
It is pointless to ask which of the moves 2 are stronger. Nobody can answer for sure. Just pick one and play it.
You do not lose games at move 1-2, you usually lose games at moves 10-20.
Having played e4 for 20 years it rings pretty much hollow now. Open positions, no particular depth. I prefer a closed approach now, posing infinite difficulties for Black. Many top stars regard closed games „richer“.