The Helldivers falloff wasn’t really because of lack of content, though that may have played a small part. The two biggest issues were the “PSN Incident,” and the fact that the game is just not balanced at all. Lots of bugs that aren’t fun, weapons that were actually fun to use getting nerfed, and AI that felt both unfair anf unfun. Allegedly it was caused by a bug AH said they fixed, but it didn’t really change much.
Basically, AH said they were making changes and when the patch came out either what they did wasn’t fun, or it didn’t actually change anything.