There are a few ways for old store pets to come back. But unfortunately there is too much greed involved.
If you were here in the beginning, you would recall that Tess wished for store pets to be available to those that could not afford them or missed out on them. This is where a few December 18th re-released store pets came from. It was halted almost immediately. The reason given was that, if you can get the pets for free, why spend any money buying them?
The large rejection to this changed her mind. And now she takes the stance that they will never be for free or resold again. To make things more reasonable, they have been deemed not official collectible pets. I would not be surprised if they get removed from the archives one day.
It also does not help that the few that spend the most are in charge. If your biggest sponsors threatened to leave cause you wanted to change store pets to always available to buy (like some store items), you would ditch that idea pronto. No reason to turn down thousands of dollars over a way to ensure everyone can get them for a short time until the mounting server bills cause a site shutdown.
Also, people want their value to go up to get those dream pets they crave but could not afford. So few support pets that do not grow in value. No one wants to spend $5 on a pet that is effectively OMGSC for always being available. It is why items don't get rarities. Its not the fluctuation thats the biggest problem, its that people focus too much on rarity and would skip on buying cause "its not worth it".
Now in theory, the next store pets could have a disclaimer that says "buying this pet is not a guarantee it will never be in the store again." This would remove the greed but take away some incentive to buy it. However this has been implemented successfully in a popular game called Fortnite. The team never promise the items brought will not come back again. Unless its specifically retired, the majority of items will be back in the shop. But it may take months or years and it will only be there for 1 day... And they still make a killing. So if Tess does decide to make them more accessible and less like how they are now, this method (or something like it) is a good possibility (even if their price goes up as a sort of compromise).
After all that I have seen with rarity drama, maybe its not such a bad idea to change this.
***Note that this is just my opinion. My facts and ideas my be completely wrong. And I am known for being wrong alot!***
P.S. I suggested 2nd or 3rd gen store pets only available in the store (created from breeding earlier store pets). It is unknown if this will ever happen. If it does, it may be the closest thing you could get to getting an old pet if they get too rare to trade for.
Hope you manage to get them before they get unobtainable. I already gave up on some.