Why? I would love for the general public to be able to connect to Fediverse. I don't love Meta per se, but let be real here, the majority of people are not gonna sign up for random instances that run by someone they don' know, to interact with a very small and niche subset of people, with a risk of it being shut down by that said person due to whatever personal circumstances they may have. It's the biggest obstacles when convincing people to switch to Mastodon.
If this rumor is real and Threads takes off, you can still stay in whichever instance that you like, but now be able to get updates from the artist, the experts, educators, politicians, the influencers that would not have joined the Fediverse otherwise. And more importantly, more people now can get updates from you. Reach might not be something you personally appreciate but it's very important for content creators.