I wouldn’t count on 50/50 split. I imagine it would probably be closer to 90/10 if threads is seen as enjoyable.
A lazy search showed currently there are 6.5 million users in the Fediverse. A similar lazy search told that Threads reportedly had 30 million signups in the first 24 hours.
Even that makes me feel like we’d be in a hidden away corner of the Fediverse just based on sheer size.