You're getting legendary weapons. I'm tired of getting Etheric Light every week when I have 30+ in my vault, but not every reward can be top tier. That's how RNG works. Nightfall is the one of the most challenging activities (especially when doing it solo) and it rewards you with an increase to exp gains. Everything else is subject to RNG, and it was way worst when Ascendant materials where on the table. While replacing standard legendaries with Nightfall specific gear sounds great, removing them will hurt the drops as we will only have coins, motes or exotics. No one likes to receive 9 coins for a Nightfall. Especially when you've already hit the cap and you have a really big stash of coins...