1.) Thunderwave is not as purposeless as you assume, the push can propel multiple objects, which can do damage when colliding with creatures or other objects. Someone demonstrated one-shotting the Adult Red Dragon in the Githyanki patrol by using Thunderwave to push a shit ton of objects down from the bridge and on top of it.
2.) Your explanation of how Darkness is supposed to work is actually incorrect. Darkness explicitly mentions that creatures with darkvision cannot see through it, so BOTH attacker and defender are functionally blind when attacking each other, neutralizing any disadvantage and advantage on attack rolls.
