Some of the complaints I've heard are exactly about that, theplottwist...a rather barren finale. Granted, it's been so long since I played the original that I can't remember how much other life the last two areas had besides the metroids.
As the team said, this wasn't a definitive release, and that there was more they wanted to do. This could have included expanding those areas. However, now that the take-downs are happening, I can't say if it can happen now.