They're making another one, this all but confirms it.
If you were Fox, would you rather go with Ridley Scott, who has Blade Runner and All the Money in the World coming out this year, likely to be hits. Not to mention Scott's prolific career.
Or Neill Blomkamp, who's last (and only) successful movie was District 9, which came out 8 years ago. And has been stuck doing short films since 2015 because Hollywood don't want him anymore.
Yeah, Alien Covenant was a 'disappointment', but it still did better than both Elysium and Chappie.
Alien 5 is just not going to happen, not under Blomkamp's watch. Ageing b-movie actor Michael Biehn and a minor character from a movie 30 years ago just isn't going to draw the crowds. It's a wonder why Fox greenlighted it in the first place (maybe they were expecting Chappie to be a big hit lol).