I'm calling it now, this is going to hollow out twitter in extremely rapid fashion. I give twitter a couple of months once this launches, they'll do a Wile E Coyote where they walk off the cliff, followed by plummeting. Meta is going to grind the blue bird to a fine powder, not saying this as a Meta fan, just a casual observer. There's massive pent-up demand for an alternative, and so far Bluesky and Mastodon haven't…
HN disables answers after some period of time, which appears to be a couple of weeks.
I think Twitter won't go in a couple of weeks. I also think that in case of Twitter not going anywhere in a year or so, we won't be able to call your call, due to HN comments disabled.
What do you propose to resolve that?
Regarding the core of your proposal, these who use Twitter and these who use Instagram, are different people. Twitter is primarily text based, Instagram is primarily picture and video based.
My prediction is that there will not be huge (dozens of percents) outflow from Twitter to Threads. I also think that there won't be noticeable (dozens of percents) use of Threads by Instagram users.
Instagram is already a social network of people with needs different from social network like Twitter.