The mode would never get played. Seriously. Unless the payout is massive disproportionate compared to Adventure mode all you're really doing is throwing ins a "bonus" to play a mode that is fundamentally worse for progression than running whatever farming you should be doing.
So you end up with campaign in the same place it is now. Ignored. You're talking about 100 votes on r/diablo. What fraction of the active playerbase is that? What fraction of the r/diablo userbase is that. There's 160k subscribers to that reddit. Is it worth the dev time to put effort into 1% of the playerbase, is the end result even going to be compelling,
At the end of the day if you want to play Campaign as it stands you can. Your suggestion boils down to "You have to do this stuff to have the best character, and it takes massive amounts of time, and you have to everything in order, and some of these you only get once and those bonuses are earlier rather than later".
Of course then you'll have the groups who maximize speed running in order to cheese things. You've locked legends to a specific mode and behind a 1hr (maybe?) time gate for single players while groups will be running through in 30 minutes or less. It's all the same kinds of issues blizz has been actively removing form the game.