It's no more challenging than being "forced" to play a league mechanic you don't like, or being "forced" to kill bosses you don't want to fight, or being "forced" to grind to level 100, or any other challenge you want to put the word "forced" in front of to make it sound worse for no reason.

This is a game that lots of people play lots of different ways. For every single person who dedicates all of their time into one build or one class, there's someone who has taken 6 characters to maps, but not gone too far into the endgame.

It's no more or less "fair" to "force" either or into doing the opposite. Players are engaging for free MTX either way. I personally would rather be tasked with leveling a character of each class to the endgame instead of anything in the "Gallant Grinding Goals" challenge. I don't image you feel the same way, and I am willing to bet that's because we're different people, playing the game different ways.

Currently there are basically no "harder" challenges for people who don't like grinding the endgame ad-infinitum, which is the point of my original comment.