Rivens do not cost plat. You can get the best one plat free. While as someone mentioned above the kubrow fur thing costed exclusivly plat.
Rivens use rng yes but you do not need real money
And saying player trading isent a good argument since you can trade with other things then plat. And also way to much plat since its not controlled by the devs.
Relics are like loot boxes but you do not need to buy them for plat. You can but since most players drown in relics who actually buys them?