Queen's Gambit decline

Hello there. Why play 1.d4 instead of 1.e4 until you reach master level? My coach told me that. He said that it is good to play 1.e4 because 1.e4 openings usually lead to sharp games (Loli-Attack,Fried Liver,Traxler Counter Attack,Marshall Gambit in the R.Lopez ,Two Knights etc) which playing these "sharp" openings may help improve one 's attacking  and tactical skills aswell as general understanding of the game. They improve tactical and attacking skills as mentoned above because there can be found different types of tactical motifs in these openings aswell as the middlegame stage of them. Example: Bishop Sacriface in the traxler counter attack (Bxf2 by black). Improvment of general understanding can be found here again,as mentoned above because the openings like the ones mentoned above are classical. Meaning that they follow the principles of a chess game (control the center,develop your pieces,castle etc). That is why i think aswell as many others that those types of openings are best for beginners. Here is a funny situation (This is only an example!) You teach your student that is important to control the center with your minor pieces, but the first opening that you introduce to him is the KID (Kings Indian Defense) which is a hypermodern opening, meaning that later on it counters white 's big center. At this point the beginner will be confused. Introducing him to the italian game with examples will make understanding the concept of center for the beginner player much easier, since the opening follows the principles. Another example is the  Caro-kann. 1.e4 c6 , now you just told the student that the center is very important and you should fight for it in the opening with your pieces first of all. But the caro-kann does not do that, it fights for the center with pawns, which again may confuse the beginning player and he may also learn something new. By this im not trying to demonstrate that these openings are completley bad and you should never play them. Im saying that is better to play openings which are simpler and don't require much theory. By this you will strenghten your opening understanding, not opening knowledge. Opening understanding explained simply means knowing general strategic rules which will help you get a good position out of the opening. While opening knowledge means knowing concere variations and moves. I think that it is much better to choose and play simpler openings,rather than cram in 90-moves theory of Sicilian Najdorf or Botvinnik Slav (There are a few advantages of the queen 's gambit declined as black, one is that it is very solid and the disadvantage it is that is very calm and as mentoned when picking openings they should be attacking and classical). So next time when choosing openings check if they are:

1.Attacking

2.Classical

3. They do not require forcing theory

Well, all i can say is good luck in futre and learining chess.