As promised in the "keiki-paste" thread, I thought I'd offer some practical evidence of the interaction between roots and the root zone environment. Consider this scenario:
A plant is potted up in nice, fresh medium, and gets great care, so it grows big and strong, sending its roots deep into the moist, airy medium. Those roots have "tailored" themselves on a cellular level to function optimally in that environment.
Time marches on, and that potting medium starts to decompose, breaks down into smaller particles, and becomes more and more compact, holding lots of water and starting to restrict air flow to the roots. Additionally, minerals from your water and fertilizers, plus plant waste products are building up more-and-more in the medium. The environment has changed, but the roots cannot.
The green, vertical arrows mark the nice depth that the roots grew originally, but judging by their color and condition, are failing in that changed environment.
You'll note, though, that there are new roots that look entirely different (see the yellow circle) that have grown from the ends of those old roots. Those nice, plump roots have grown with their cells optimized for that environment, even if it is an environment that is bad for the original root system.
One might surmise that the plant will therefore be OK, as it now has well-functioning roots. Unfortunately, while those newly-grown root segments may be fine for that environment, the older root system is not, so will continue to deteriorate, ultimately completely separating those new roots from the plant, leading to its demise.
That is why, when considering repotting a plant, it is best to do so just as new roots are emerging from the base of the plant.