One concern is that since threads has a massive userbase and similar volume of content, it is basically a full reservoir and when it starts to federate content it will be like the dam bursting. Even if there is a need for a user on lemmy.world to do something to start federating content, like subscribing to communities, but all it would take is a bot that subscribes to a bunch of popular content to both fill the All feed and prompt a massive number of API calls.

If the hardware is up to the task, the other concern is the threads content overwhelming existing communities, and since a bunch of meta content is bot driven and malicious that would be a crazy amount of moderation that is likely needed to keep it from causing issues and driving away the existing userbase.

Lemmy is niche and therefore is heavily populated by techies but more specifically lemmy is open source so these techies are specifically the type who like open source stuff. Threads and the corporation responsible for it have a financial incentive to oppose open source projects like this. So the community most ideologically tied to lemmy want nothing to do with it. They want to preserve their space that they have made as free of the influences of capital as possible. The very existence of threads is a threat.