Okay okay, I'm going to explain this simply to all of you so you understand.
Good A is a substitute for Good B when there is a decrease in demand (or supply) of Good B.
Good A is a complement for Good B when there is either an increase or decrease in demand of Good B.
Lemonade and Iced Tea are substitutes of each other. Both serve the same purpose for the majority of their consumers--refreshment. If demand for Iced Tea goes down (maybe because its price went up) more people buy its (cheaper) substitute, lemonade.
Gaming mice and video games are complements. If demand for a competitive shooter goes up, then so does the demand for gaming mice. Likewise, if demand for video games as a whole goes down, less people will buy gaming mice. They complement each other.
Valorant and CS:GO, as two goods, aren't as simple as the above examples; the reason being, there is no single appeal that defines the consumer for either game.
You could say both are substitutes because most players of either game just see them as nearly identical tactical shooters, sure, but Valorant and CS:GO but there's an asymmetry between each games' playstyle.
For example:
One person might like Valorant because it's more ability focused, and they don't have to have insanely good aim to be good at the game itself. If Valorant were to die, it's highly unlikely that they would switch to CS:GO.
Likewise, another person might like CS:GO better because they prefer the heavy focus on aim, and don't want their screens clustered by abilities. If CS:GO were to die, I doubt they (alongside many, many, MANY outspoken CS:GO players) would want to play valorant.
That being said, as I mentioned before, it's all a matter of each game's consumer-base. The key to the issue is finding out what percentage of each game values in their respective choices, and why.
Sure, there are aim fundamentalists in Valorant too, and if CS:GO were to explode, they might switch over, but that's not to say the entirety of the community would switch over.
Unsurprisingly enough, there are a lot of strategy-minded CS:GO players that would find themselves like fish in water if they switched over to Valorant (like FNS did).
Going forward, with an influx of new players to either game, some might find that whatever their choice was initially was not the right fit for them, and end up switching to the other game, while those that do enjoy their picks will simply stay.
So, the final ruling is this:
Valorant and CS:GO serve as substitutes for portions of each game's respective player-bases, but just like a 0.01$ increase in Lemonade prices wouldn't automatically make everyone stop drinking lemonade altogether, unless CS:GO 2 REALLY kicks off in popularity, it won't affect Valorant. Furthermore, Valorant will still retain a large, dedicated core community unless they really fuck up what makes their game unique (horrible balancing of abilities), but less they want to lose all their customers to Valve, Riot will try extra hard not to do that (healthy competition).
They aren't complements, they're substitutes and their developers are therefore competitors (but that's a good thing.)