Everything you write makes sense. I’m just not entirely sure that adding affix slots will solve the “dullness” the exact way you described.
In a game where you don’t have deterministic crafting, having more affix slots serves the purpose that you are more likely to find a valuable item. You are happy if you find an item with 4 good stats and just accept that 2 trash stats on the item. Because you can’t do nothing about it.
Would you accept these trash stats in LE, too? Or would the goal be to have 6 fitting affixes?
I think it would be the latter. Just because you already have 2 good suffixes would not make a trash suffix ok. You would accept it for the leveling phase and along the road. But for endgame gear you would aim for 6 perfect fitting affixes.
The “filler” affixes don’t get valuable just because now you have more slots. These more slots will be filled with more of the valuable affixes. Imagine 2h weapons with 3x flat t5 melee damage.
Resistance will stay no.1 choice on suffix slots. You will cap res with less items or you go for 2x res and 1x endurance affix on every item possible.
In the end the choices might not be more diverse. It just adds player power, that has to be balanced.
And longevity also may end in tedious gearing. It would take a lot longer for people to get t30 gear, as you said. For some it is longevity. For others it’s just the carrot on the stick that is always visible but not in reach. This can end in complaints that t30 gear is hard/impossible to get. Everybody will want to have 6 perfect affixes, but only the lucky ones and the hardcore layers will be able to get it.
This is my fear when changing the system. I’m not against more diverse gearing options. But I think it not just adding slots.