Given the stories we just went through with Shadowlands, each of the covenants we met up with could have valid reasons for fighting alongside the mortal races.
- Kyrians just had their entire way of afterlife changed due to a mortal showing them the importance of keeping one's memories & one's self identity. They still have much to learn about the world, the mortals can teach them quite a bit.
- The Maldraxxi are always up for a good fight, and the mortal races of Azeroth were instrumental in the biggest fight they've had yet. Plus with the Primus having such a strong military mind, he'd see the wisdom in continuing the relationship with the mortal races in case further help is needed.
- Given that Elune tried to give her favored people to Ardenweald as a gift to her sister, the Winter Queen well could choose to repay in kind by sending guardians to help the remaining few of Elune's favored children.
- The Venthyr have now with the new Arbiter lost their greatest asset, a place to send souls beyond redemption. They have also lost their leader & creator, now being run instead by his son. To better learn how to bring repentance to those most lost, they can learn much from fighting at the side of mortals.