None of the evidence we have currently goes against Calyrex and arceus existing together. First of all I would like to say that pokemon being overpowered does not have any role in making them cartridge legal. If you can get eternamax eternatus legally in any pokemon game which is not single player it can very well be used in the anything goes meta. However it is a confirmed fact that using 2 mega evolutions in the same team is not possible. Basically while making the National Dex metagame they cannot go against any established fact no matter how it affects the metagame.The evidence just as much suggests to the fact that there will never be a cartridge legal format in which you can see Arceus fighting Calyrex, yet here we are.