This sounds awful, they already said we are paying off a debt, adding an Upkeep to that would really strain the fun out of it.
It honestly depends on what the developer chooses to use. The monsters needs something to do other than just being sold via requests.
I will use Breeding farm as an example of what not to do. Currently the gameplay loop for monsters is just breed a species, level it up to the requested level and then sell that monster. If not the requested monster type then the only way to get rid of it is just abandoning it. Traits, stat rankings, and stats do not matter. Your only limit to monsters is how large your barn is but you only have 2 methods of making money (selling monsters, working in the bar).
Now logically there should be upkeep for the farm. Resources or money. If money then you need methods to make more money. But Alternatively we could take the concept of the farm and have food being grown. Initially player could do some work on it or unlock it with say a plant monster and then to increase base amount given have monsters trained to handle the different jobs depending on how far you want to go into that.
If you are playing with the idea of post apocalypse or some element of danger then a security force could make sense to stop raids on your farm, be used to hunt down more dangerous monster types to act as your starting point, be a regular request from a client (military) or some form of competitve fighting.
The point is the monsters either need some additional value in the gameplay loop to justify them as the gameplay loop. It could even be something like releasing into the wild to fix an ecosystem.
This said NPCs and the playable character should have some value in that loop if they are to be included in the breeding gameplay loop. It could be something as simple as them having specific unique traits (which ideally if going down this route should have traits degrade over generations). Maybe an NPC is the only one of an NPC able to breed with certain types of monsters (meaning they unlock the monster type and your first starter of that monster).
The things is a bad gameplay loop or unfinished one will just have the game stagnate once you get past the early game.