That just forces 8 players in the candidates to do their preparation BEFORE the tournament.

Economically speaking, that won't be enough time for advertising, setting the venue or getting sponsor up either. It matters who plays in the WCC match before you can do either of those things. We still don't know when or where the match for this cycle will happen even now.

Not only does it force the 8 players to prepare before the tournament, it forces the champion to prepare against all 8 players, making their task even worse!

Worse or, and that would be the point of the change, entirely different. You would do some study of all possible opponents, but you obviously can no longer do so much focus on the single player. Which means, the better preparation would just to prepare your best chess against whomever you are going to play. Which could be much more fun and make the games more interesting, as there is much more space for tactical surprises.