Yeah for me they just force pair them for not having à "triangle amoureux" with the MC his gf and the blonde girl. That pjss me so much i dont want the see the rest of the story
Forcibly pairing side characters is a hundred times better than a harem around the main character. On the other hand, it's also stupid if the MC pair remains the only pair forever and the side characters are doomed to watch them in envy and endlessly whine about being single, while the MC pair keeps flirting.
Besides, Li Xin has actually been looking for a romantic partner for a long time now. She even soured her friendship with the blue-haired mage girl over a dude, and would have chased after the MC if their age difference wasn't so huge.
At the end of the day, this series is quite peculiar among xianxia as it is. Usually the romance is completely slapped on, and works so that the MC falls in love with a girl during the early chapters, then is separated from the girl for 90+% of the series, only to be reunited near the end. So, the girl basically isn't there at all and is acting as nothing but an abstract motivation for the MC to push forward. So, if this series does the opposite in that, it might as well do the opposite with side characters and allow them to have romances as well. Also, this manhua is terribly abridged from the novel, as you must have realised already.