There is a technical limit to the game, when it would be too resource consuming for platforms to run. It is already not optimized well for large zoos, but we are far from it.
I think you're mixing two kinds of optimization flows here.
On one hand you have the ability of the game to load all the available content you can use to build your zoo. That is part of the optimization that can influence adding more DLC content to the game. There are a multitude of ways to handle this issue, a lot of it has to do with dynamically loading in assets in the game the moment someone is scrolling through the construction tab for instance, and the vast majority of solutions to that are pretty extendable.
Now of course at one point, as with anything in development, you reach the limit to what you can do with that. So at that point but preferably a little bit before you hit that point, you stop adding new content to your game.
Now the ability of the game to handle large zoos on lower end systems ( as large zoos run surprisingly well on higher end machines) however is irrelevant to adding more DLC content to your game. Planet Zoo is a game in a genre of similar near limitless sandbox games. This genre deliberately makes the choice not to put too many limits upon its players in what they can do, at the costs of the players having to invest in higher end parts/computers once they're hitting lag. It is not a choice everyone can agree with, but it doesn't make it a bad choice because of that. It is simply a choice that is part of this kind of game.
That obviously doesn't mean that they don't look into optimizing the game so you can at least go as far as you can, but once you hit a hardware limitation, you are on your own. It's the nature of the genre, and clearly enough people are a fan of it because it hasn't died out yet.
If datamining points to anything, the next full pack will be either a rainforest theme (capybara--a South American Pack part 2 is possible but I don't know if Frontier would go for it) or an East Asian theme (unused hot spring enrichment and certain props, maybe focused on Japan and maybe headlined by the red-crowned crane rather than the Amur leopard).
Those animals and scenery ( capybara was even a sign, not an animal) were tagged with tags from themes of previous DLCs. Just like how an emperor penguin sign was found during the beta which later on ended up becoming a king penguin, these things are much more likely to be scrapped content than indications of future content.
Doesn't mean a capybara isn't going to happen, I think it's a logical animal given the meta-whishlist, but the stuff you mentioned isn't solid enough to point to something in the future.