This would push users toward local posts (especially on smaller instances) while still supporting a better distribution of user engagement over the threadiverse. And if you don’t trust that it was implemented correctly – you can simply check it by counting the amounts of posts against your chosen probabilities.
It’s not fair to say that something is a bad idea and not even say why. Could you please elaborate on why this is a bad idea? It is specifically toggleable -optional whatever. I think it’s a solid idea.