I have read that after some time, substrates do become 'stale' with oxygen-robbing bacteria making them unsuitable for decent root growth.
by 'oxygen robbing' i presume you mean anaerobic bacteria? If so they are actually helpful in substrates. If the bacteria in the substrate are aerobic then they can keep elements such as iron locked up in their oxide precippitates which are of little use to plants, also, anaerobic bacteria can convert nitrate into nitrogen therefore keeping nitrate levels low, and it also means that the substrate will more happily accept electrons, so therefore positive elements are attracted to the substrate and it brings K+ and Ca+ etc into the substrate.