If gm crops can decrease pesticide use, why are environmentalists still concerned about them? (scenario gm)

Because gm crops can't replace everything. yes even they can make a better species and increase the productivity, but some countries or farmers might not have the technology to replace all the crops with gm crops, and they still have to rely on chemical fertilisers. chemical fertilisers can be deadly as if u overuse them, it can pollute the water and leading to euthrophication, where it can kill sea animals