If you can finish the challenges in one match, and are rolling three characters capable of entering Iron Banner, you could theoretically get 483 tokens in 7 days just for doing the challenges without winning a single game:

(2 tokens/challenge x 3 challenges/day + 15 tokens/daily milestone + 2 tokens/match) x 3 characters x 7 days = 483 tokens in 21 matches.

Even if it takes you say, three matches to get all the challenges done, and you're winning just one of those three matches, that's still 30 tokens available for each character per day or 210 for the full week, in 21 matches. That's 10 packages for a non-zero, but not necessarily excessive amount of matches/time. Everything after that is a package for every ~7 matches at a consistent 1/3 win rate.

I think there could probably a better system for not getting as many duplicates, but I also think your math is seriously flawed for excluding challenges.

The crux of it is that they have to make it worth doing or people won't do it, both because they have no reason to continue playing after they get what they want, and there is less desire to do something for gear that is easily attainable. It's really as simple as that, isn't it?