Any Dutch player (and any white player who is prepared to face the Dutch) will tell you that the position after white castles is the standard one and that the immediate aims for the two sides which define the course of the game over the next few moves are -
- White: force through e4 (and prevent black from playing e5 or f5 advantageously)
- Black: force through either e5 or f5 and prevent white's e4
If either side achieves their aims they have "won" the opening. They will have the initiative with good prospects for an attack. Of course, with good play on both sides this doesn't happen and the struggle for the upper hand goes on into the middlegame.
.7. ...c6 is very unusual / bad because it does nothing to prepare e5 for black and nothing to fight against white's e4.
.8. d5 shows a complete lack of understanding since it gives black e5 for free. After this the position is at best equal for white. Dutch aficionados will tell you that black is better. He has achieved his dream position out of the opening.
.8. Qd3 would have punished black because now the only way black can prevent 9. e4 is by 8. ...d5 which transposes to a bad Stonewall Dutch a move down and with the bishop misplaced on e7. All the dynamism has gone out of black's position. He has given up all hope of pushing e5, given away the e5 square to white's knight and has a dour prospectless middlegame ahead.
PS I hate the fact that I had to put some random character before 7, 8 and 8 in the last 3 paragraphs to stop the formatter turning them into an enumerated list. Bad enough in itself but it "corrected" the last "8" into a "9"! Let me choose when it is an enumerated list and when it is a list of moves.