Do note that you can't currently message users on other instances from kbin. The button will still be there, but it will bring you back to the homepage. I believe Ernest said something about it being "unimplemented," so I'm assuming cross-instance messaging will happen someday, but is probably not a high priority at the moment.
And in fact people who who want to interact with the 140million ish Threads users currently have one option - join Threads. With federation I can communicate with Threads users without joining Threads.
What if the defederation happens in the other direction? Defederating an instance is a lot like banning a user, and I'm not sure if there are any mainstream social media sites that I haven't heard abuse their ban system. If other instances start becoming more popular because people want to use them to talk to Threads, that gives Threads a lot of power over which of those instances are allowed to thrive. In the worst case scenario, it could easily kill an instance if too many of their users were there for Threads and Threads decides to cut them off.
A fediverse that is popular because it can talk to a centralized app doesn't sound like a particularly healthy fediverse to me.
Is the microblog tab actually that important to most people, that the instance could become dependent on Threads for dominating it?
I don't think it could put the entire instance in jeopardy, but personally I think the microblog tab has a lot of potential (there have already been strides to incorporate it more), and I'd feel a lot less positive about its possibilities if it were full of content from Threads.
When people go to Mastodon, Kbin, Lemmy, Firefish, Misskey, etc., they do so knowing they're going to the fediverse. When people go to Threads, most do so because they have an Instagram account.
This is my main concern.
Personally, I don't care if the fediverse grows. I just care what it grows into. The fediverse has a nice community at the moment because everybody on it made a conscious decision to be here and not somewhere else. Threads users will not have made that decision. Furthermore, they'll outnumber the rest of us enough as to have no incentive to try and fit into the preexisting community here (which isn't helped by the fact that they've already been their own isolated community for awhile).
I second all the suggestions except the currently reading/online one: in my experience, seeing "currently reading" statistics just makes dead places feel more dead. It's also not a useful statistic: it doesn't tell you how good a post is, or provide any information about the poster, or even show how popular it is (because you can't really extrapolate about the average engagement from only one datapoint). You can't do anything with the number.
It's usually easier to search via external sites (https://lemmyverse.net/ and https://browse.feddit.de/ are the ones I've been using). You'll still have to manually copy over the magazine name in order to subscribe for it, but you'll get a wider variety of results without having to go to each instance individually.