In this way, Witch doesn't become stronger.

I disagree. Junkers are rarely ignorable but sometimes they are. In this very situation in which you ignore a junker in a 2P game you would perhaps not do the same in a 4P game as the junk pool is larger. So not getting that Marauder in a 2P game might be fine, you can only get a max. of 10 Ruins. But in a 4P game, if you are the only player without a Marauder, you could end up with 40 Ruins.

This isn't correct, because all the other opponents are getting some of that junk as well. Just look at the math in a game where your opponent buys Witch and you don't:

2 player game: You end up with 10 Curses, your opponent ends up with 0.

3 player game, 1 opponent buys Witch, the other doesn't: You end up with 10 Curses, so does the other opponent.

3 player game, both opponents buy Witch: You end up with 10 Curses, each opponent ends up with 5.

The point is that each time a Witch is played in 3 player games, 2 Curses are given out (from the pool of 20). So you can't end up with all 20, or even more than 10, unless some players are using cards like Moat also.

In fact it's much worse in a 2 player game; where you have 10 Curse cards more than your opponent. In a 3 player game, if both opponents buy Witches, you only end up with 5 Curses more than your opponents.

4 player math works out the same way.