Not if their communities and friends leave for threads.net because by the time the defederate occurs most of the communities either are on threads.net or have threads.net equivilants.
You can't have a community without the community part of it.
Why would their existing communities and friends leave for threads.net if they are on Mastodon to begin with? Most fediverse users I find seem to be pretty passionate about the platform, I doubt they will leave just because their friends are on Threads.
I say they can just make a Threads account for when they want to interact with those friends. It isn't that hard. I use different services to check my bank balance or to pay my electric bill.
Because a lot of the new traffic is really less passionate about fediverse, and more passionate about getting away from Reddit and Twitter. Plus the friends/communities people will make that come from that group.
You're thinking too short-term and not after things have started to reach some normalcy again. And also that Meta is specifically trying to get in now while those communities are trying to form.
And what's wrong with people making that choice? I know some of the "mainstream" people on Mastodon right now left because of Twitter's craziness but would probably be open to having an account on Threads or Blue Sky. Why the paranoia about people making choices that are best for them? TBH Mastadon search sucks and finding accounts to follow is hard.
Because once these centralized services reach the point of defacto, they always just trying to push more and more of what they want. We're practically hitting a bubble at the moment of every social platform that took over trying to nickle and dime. It's the reason for all of this in the first place.
Facebook - Well it was always crap.
Reddit - killed all the forums, now trying to own all the text as content, making lives harder for it's moderators, and continuing not to pay them for the clearly thankless job. Quality of posts has degraded.
twitter - messed up the entire purpose of their checkbox, keeps trying to find various ways to make money, entire quality of platform quickly degrading due to server migration
Youtube - Has started pushing for more ads, and punishing users who don't watch said ads.
People think about the short-term too easily, and then just kick and scream when it's already too late, and moving requires a ton of coordinated work, and "putting up with", making new things work, because everyone let the last thing get screwed over.
The constant cycle of letting products get ruined and moving on gets annoying after awhile.
It's too easy to say "why not?" and very difficult to word "Because it always ends the same"
People have a tendency to flock to centralized services. Even on here the majority of activity takes place on Lemmy.world, the fediverse already has a potential centralization problem.
With your point on YouTube, a fediverse version of YouTube that has a similar level and amount of content is impossible due to hosting costs. I have yet to see anyone willing to subsidize a fediverse version of YouTube.