>>89022506The first game had 3 maps that were just 4 arenas each with a few points to stand on and nothing more
and while it was helped by the augmented mod, I've managed to log 900 hours just by replaying those maps
8 missions, even if they're not as long, should be plenty of content to enjoy in the same way, especially with twice as many classes as last game had (I might not like what primaris has done to marines on the tabletop, but I have to admit they make for better video game classes). I've still not played all 8 actually, despite some 70 hours in the game so far, and I've avoided the highest difficulty too. Currently working my way up to a really good power fist.
So yes, replayability should be king here.