So, if the noble's guild abuses their subcontractor the commenor's guild that much, shouldn't they quit their common efforts? I know all the well paid contracts from nobles's and merchants go through them, but are they worth it? Or are they that necessary? If the commoners guild could make due with normal contracts and dungeon looting, they really should cease to work together. On one side it will be hard to sell the minor loot to merchants, as they would boycott them first, but will the merchant's guild really be able to satisfy the needs of the town? The noble's guild wo't help them to loot the stuff they need. They would have to import stuff to hunger the commoner's guild out and hire commoners behind their back, yet still...How long will they hold out to pay them better then the commoner's guild(cg)?
On the other side the noble's guild (ng) will face some harsh challenges, if they have to face the dungeon alone. They will fail more quests and be pressured from all sides. Sure they will hire some merchenaries, but that is more expensive then a fair price for the cg. And then noble's will increasingly die.
So if the cg stops cooperation, it will be bad for the commerce, which is worst for the merchants and therefore mg, and it will tarnish the reputation and hurt the ng. It would boil down to, if the cg is able to survive that long (stops their adventurer from quitting and joining the merchant guild or a new fake cg)
Then again, after nobles dying and enough complaining the king might force the cg to cooperate...