"Maybe, until these pets reach the saturation point for the target demographic. And then they become cheaper and more obsolete, down to 3k... then 1k... then around 500 gold a few months from now. "

Where do people get the idea these are going to reach 500 gold just because the biggest demand will wear off? As the demand goes away, so will the supply, naturally. In order for your theory to come to pass, people would have to see prices that low, yet still decide to buy the pet to sell it. The sheer terribleness of that deal is going to stop people from doing that.

The whelplings, for example, are still pulling in around 10k gold. I'd imagine these guardian cubs are going to continue to pull in the same amount into the distant future, because as demand dissipates, you're going to see people purchasing cubs to sell them just about as often as lucky farmers find whelps on corpses.

You're correct that there's going to be less of a market as it saturates, so this particular pet is going to cease to hurt the gold sellers -- but I think that's going to be due to the unreliability of quickly selling one or more at 10k...not because of the complete inability to sell one at that price.