Seeing how Goh caught a Suicune which is a popular legendary and is allowed to roam. Couldn't this scenario be a factor of Ash owning a Darkai but allowing it to roam while calling upon it when its need for important battles. This is how many of Ash's powerhouses and rare pokemon catches have been traditionally handled , such as charizard, naganedel etc.
The fact that the Anime is allowing both Ash and Goh to catch rare pokemon like Suicune and Dragonite shows that the cycle of old trends are being broken. Ash hardly uses dragonite, except for very tough battles and the same reasoning could be given for Darkai. Based on the forums debate, some fans just assume that pokemon that are considered very rare are still off limits for potential captures simply because they feel it wont work in the story. Except, the focus of this arc is Ash battling tough trainers around the world, who are capable of challenging mythical and legendary pokemon. As the series progresses, we are now being introduced to more trainers owning legendaries, especially with the introduction of the Urshifu trainer.
To me it seems obvious that Ash will eventually get his own first legendary this season, and perhaps catch one mythical along the way. It was my assumption that since Darkai is a very popular mythical it seemed like an opportunity for Ash to have one. I mean Zarude is also a mythical and there are more than one of its species living in the jungle. Melmetal is an extremely rare mythical, havin not been seen for 1,000 of years yet Ash gets to keep it.
The series has shown that mythical and legendary pokemon are not often singular species, being one of its own kind, and have been owned by other trainers which apparently isn't seen as too shocking in the anime. logic tells me, that a trainer capturing one does not seem to jump out as a surprise of it being too unusual to anime world, since there are sometimes more than one of other rare species of pokemon in the world.