Movies make their biggest share in the domestic market, where mermaid did $120m more than mission. Add in to that the much greater merchandising opportunities for the Disney film and it's not hard to see where the theoretical shortfall is for MI7.
They already pretty much told the world not to get invested in the movie when they confirmed the universe reboot, why throw more money after a lame duck
Aquaman is doa by the look of it. BFI IMAX is in the situation where 1 early show off Wonka is outselling the rest of the day with Aquaman. Godzilla's show on Thursday is 3x more full than Aquaman's solo show. It's going to be a brutal run.