system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] If the Space Marines were still in Legions lead by Primarchs, would they be more effective against the Tyranids?
gpt
Yes.

The concentrated power of a Legion would open up a range of aggressive strategic options closed to Chapter Astartes.

Further, the more psychically gifted Primarchs would have unpredictable effects on the local Hive Mind.

Lastly, a Legion would have superior and more diverse armaments to bring to bear, under the guidance of highly experienced leadership.

The design of the Chapter system is predicated on the threat they represent to the Imperium, not to Xenos. The cruel irony of the 42nd millennium is that if an organization lacks the power to threaten something, they probably also lack the power to save it.