There’s something I don’t think the Forerunner trilogy covered about their economy; mainly, did they have a post-scarcity economy? That is, was all production automated, without the need for labor. And if so, did many Forerunners suffer from apathy, as they had everything provided for them without any effort on their part? Did that contribute to their downfall?
I highly doubt that any Forerunners actually produced anything. We have seen several examples of automated manufacturing technology from the Forerunners. Did this directly impact their demise? Possibly, though I think it more likely that their apathy was produced by their massive lifespans.
Or, their apathy was a result of having everything they need handed to them without striving for it, and living indefinitely without a purpose. The Forerunners aren’t like the Federation from Star Trek, who get what they need, but still strive to better themselves, as they are small fish in a galaxy full of sharks. The Forerunners, on the other hand, were the top dogs for thousands of years, and had successfully pacified much of the races of the galaxy under their rule. Living indefinitely, in a galaxy at peace, without a purpose, and with no need to strive for anything, would no doubt create a large population of apathetic people. It happens in real life, and fiction is often a reflection of what goes on in the real world.