Demand always increases because more users join and want to try for that fabled "complete collection".
Heck even I gave up on the idea that I could maintain one after successfully doing so for three years.
So many pets get released each month and the more litters that are released, the easier it is to miss outcomes.
When users decide to quit, they more than likely just stop logging in rather than donating pets to the pound in case they ever come back.
I will say, I'm always surprised and amazed when I see people gifting for no other reason than to help out.