The vast majority of those users want/need a bigger network than is currently available on the Fediverse to get the breadth and depth of content that was on those sites.
Disclaimer, i havent used Threads. But everything i've seen from it was just influencer spam, grifting and corporate twitter. I dont think i want that kind of content. Quite the opposite, this is the reason i chose the fediverse.
My hope is that Threads sticks with a shitty algorithmic feed and bombards people with corporate bullshit, and its users find out that independent instances exist and will give them more control.
Why would anyone sign up for those instances if they can just look at that stuff from Threads? Furthermore, it would actually make things more difficult, because explaining the fediverse to people that are coming from a corporate social media but that have already had fediverse content is just going to turn them away. It would be far more comfortable for them to just continue using threads.
Thirdly, it would also influence the federated instances. All the influencer spam and brand bs thats going on over there would also end up on the fediverse.
And lastly, we dont have to win over every user and every bit of content. The fediverse isnt some VC funded social media that requires unlimited growth. If there is nothing good to grow into, it can just stay the size it is and be fine. I dont get the constant arguments for growth if the new content would be the worst social media can offer and the users would be facebook tier grifters.