Well in Helldivers 2s case, its not helpful that they picked to use a dead game engine. Autodesk Stingray has been dead for a while.
Also, I might agree except that solo indie devs in their basement can add many basic features in 6 months time, not just one. I get that some features, like new maps, mechanics, or characters take time. But for example, when a game mechanic already exists elsewhere in a game but not in a different part (for example, a flashlight attachment on one gun but not a different gun), there is not a thing in the world that will convince me that would take 6 months to add. And if it would take 6 months to add, that is entirely due to laziness or incompetence.