Of course thi will never be allowed, people won't pay for dlc if modding can bring new content....I understand it, and I'm ok with it as long as the dlc are good.
That is literally not true, whatsoever. There will ALWAYS be people scrambling to buy DLC, regardless of whether modding can add new content or not. Modding is the main thing that tends to keep games like this alive- Zoo Tycoon 2 STILL has an active playerbase, to this day, thanks to the modding community.
But people will ALWAYS still want "official" content. Even the modders. Even the people who USE mods, would STILL want to buy the DLC. Most people don't like leaving DLC un-bought on a game they own, the vast majority of people will want the full content of the game, including DLC. The existence of mods does not, and will NEVER, change that. Modding can only ever improve a game, and its lifespan.
Take The Sims for example... the Sims has a highly active modding community, CONSTANTLY adding new content, entirely new scripts, new CAS items, open world is even being done via mods for Sims 4, and yet people STILL buy the DLCs. In fact, I'd argue the VAST MAJORITY of the playerbase still buys DLC, even if they use mods. It's the same deal for ARK, and MANY other games.
Mod support 100% determines how long a game will last and retain an active playerbase. I can tell you right now that if it weren't for mods, I would not be playing this game anymore. There are a TON of model inaccuracies when it comes to animal appearance(especially noticeable with the big cats, hyenas, and wolves), and many other issues that can be fixed with modding.
Modding does the complete OPPOSITE of what you're saying- modding is what keeps players of zoo games like this interested, and WILLING to keep playing and buying DLCs. Why would I buy a DLC for this game if I'm not going to even play with the base stuff at all anymore? There's only so long I can look at the cartoonified lion before I get tired of it. If it couldn't be modded and improved on... along with MANY other aspects of this game... then a lot of people, like me, would just give up on this game entirely because we're not satisfied with it and we can't even fix it with mods.
But even with the very small, minor modding capability we have currently- we still cannot add new animals, or code new animal behaviors, or anything. Modding for Planet Zoo is nowhere NEAR the level of Zoo Tycoon 2, and you know what that means?
Zoo Tycoon 2 will outlive Planet Zoo.
People will still be active in Zoo Tycoon 2 modding communities, buying the game, playing the game, for YEARS after Planet Zoo's fanbase fizzles out and dies. What happens when Frontier isn't releasing new DLC anymore? The game will die. People will get bored and move on because PZ doesn't have enough mod support to keep the game going longterm. After Frontier stops adding DLC, that's it. No more animals. No more scenery pieces. No more new content at all, period, unless they add in actual, fully functional mod support.
If Frontier added mod support, people ten years from now would find mods for the game that they find interesting, and go buy the game FOR those mods. They would ALSO buy the DLCs, because some mods will require DLC content to function. If a game's community is dead, and there hasn't been any new content for years... people will just stop buying, and stop playing.
Mod support can ONLY ever be a good thing, for the game itself, the community, AND the developers.