I don't see how they would lose any more than they have to this point by continuing to detain them. If anything, they want people and other countries to know that they will resort to hostage diplomacy if one of their nationals is threatened.
I would even say that they lose credibility (and face) by releasing them without any sort of quid pro quo to justify it.
I dunno, I don't see a light suddenly going off in China along the lines of, 'shit, we look really bad randomly detaining Canadians, we better let them go'. Especially cause they would view having the Michaels as leverage over Canada, Canada still controls Meng's fate (to an extent), and releasing the Michaels is giving up on that leverage before the ordeal has made its way through the Canadian courts. Aside from a handful of Western nations, most everyone has been silent about it.
Ultimately I think the priority for China is to demonstrate strength here, and releasing the Canadians before securing Meng's release doesn't accomplish that.