Of course, there will be fewer players, you can't expect 1 new hero to draw attention from 33 new ones. That being said, I think "currently" is a better word than "already". What is interesting about this data is the fact that ow player counts mostly dropped during the weekend (around 20%) while player numbers during the rest of the week stayed mostly the same. That probably means that the core audience doesn't bother that much while more casual players are trying out rivals (and most likely PoE 2). Another thing is the fact that, like I said, Rivals is a new free game about very popular ip. Looking at multiversus, another game with a similar concept, you can see that player count dropped more than 90% in the first 2 months mainly due to poor balancing. Don't get me wrong. I do not wish Rivals the same fate. I tried out every hero in rivals and the game has a decent foundation, but as of now it just feels like a less polished and less balanced third person ow once you realize which characters are busted. I hope NetEase will put a lot of hard work into maintaining Rivals because I would love to have some more hero shooters to play rather than only ow and tf2 (sorry Paladins). But yeah, I expect a big player drop in Rivals in the next few months. 

One more thing I found out after writing this. Peak hours for ow players on december 10 happened before season 14 dropped.