Really good question! This is actually something we’re still working on. Decaying structures are indeed planned, not only to get resources out of the system again, but also to balance the game a bit in favor of newcomers, whose brand-new buildings still operate perfectly. And of course, players will (want to) expand their companies to other planets, so there’s always going to be the need for more and more resources. As for having to buy those resources: We’re going to test a new mechanic in the next run that is supposed to reward specialization and hence increase the players’ dependence upon each other.
As of now, there’s no super-detailed overview of the game’s mechanics, partly because we don’t want to bother with it as long as it’s subject to such big changes. For now, the Feature List will have to do, but more in-depth tutorials and possibly other learning sources like a wiki are definitely on the horizon.