@Shadout First of all we don’t know if there have been any iterations on keyed dungeons. Also we don’'t know how deep they go as far as difficulty or how they scale in difficulty. That means that a campaign plus system might need to have 20+ difficulty levels to be identical to the keyed dungeons. I wonder how many players would love another 20+ difficulty system like D3. Because we can replay the difficulty at max level on any difficulty we please.
I think ultimately only less than one percent of the people would even bother to play it. Why go through a big campaign with the same rewards as keyed dungeons when keyed dungeons will no doubt be smaller than the whole world of Sanctuary.
Look it no doubt gets boring listening to the same story over and over and over again for the one hundredth plus times. This is true for the ones that have won awards on their story. More so when the campaign is on the rails. A linear campaign is boring as hell.
Players these days don’t mind going through the campaign at the most once per character then endgame. No you don’t have to be max level to participate at endgame. PoE has proven that to be the case and it is a good idea. That means you can still make progress as far as leveling goes by playing the endgame.
When we don’t know how many affixes will exist. When we don’t know how the keyed dungeons scale in difficulty. You could have a system where a campaign plus might need 20+ difficulty settings just to be in line with what the OP is wanting.