Or they could just solve the problem both ways by simply creating an item that can be infused into any primary to give it elemental burn, so each person could have their own favorite weapons they already like just have an added bonus. If they didn't want people to have tons of elemental primaries, they could limit how many of those items you could get per account, or make the item only work on exotic primaries, or hell even make it a temporary buff item you could use, but perhaps have a cool-down waiting period like ammo synths. There are many ways that they could incorporate elemental damage into primaries, but the real reason they removed it is because it made their content too easy because everything was just a bullet sponge. Since Taken King they have added more variety and style to some bosses, and if they continue to do that to a greater scale and more often then they won't have to nerf primary's elemental factor just so content take longer and is artificially harder.