Enable free character after finishing story line.

If they do that, you would see less mtx, and since GGG is funded by your mtx, it is a conflict of interest and their need to make money is above your comfort or experience

I would willingly pay money for a free ride through the storyline.
I think $10 would be fair and reasonable, call it "leveling MTX".
Also doing another character mostly means you haven't got to gear up assuming you already have currency from your first build. So the thrill of getting better gear is gone.

Realtalk: If you can't get through the campaign in three hours on your second+ character of the league, you are the person the campaign is FOR.