Technically you can't earn weapons, everything is RNG so what you call earn is just hoping RNGesus loves you. The only way to actually earn weapons is to complete exotic bounties. Otherwise you can't earn weapons, you can only receive them randomly. Anyway, people who haven't been blessed by RNGesus and don't get anything from nightfalls or raids should be allowed to purchase at least 1 exotic a week from xur, surely they deserve that?