Yes, middle ages is more about society "style". The main economy may be scavenging, which is very unproductive and prevent to return to manufacturing economy, as scavenging will give high quality objects for hundred of years. As a result, the skills for manufacturing will be lost. Also a lot of weaponry will remain including knifes and guns (and even nukes), which may result in Long War or Somali-style world (I think that these notations are self evident). Such Long War in scavengers world will mean very long process of degradation and population decline, which is rising chances of total extinction. A lot of postapocaliptic fantasy is trying to model such world.
Some information may be here:
The Knowledge: How to Rebuild Civilization in the Aftermath of a Cataclysm
by Lewis Dartnell
http://www.amazon.com/The-Knowledge-Civilization-Aftermath-Cataclysm/dp/0143127047
A knife is a tool as well as a weapon.
I think you've got a point about the loss of knowledge if people are scavenging for crucial items.
Population decline strikes me as hard to predict. My casual assumption is drastic decline during the disaster and soon after, followed by gradual increase as people move into abandoned land and grow food.
I also assume that people will be inventing new things to suit their current conditions.
Levels of global catastrophes: from mild to extinction
It is important to make a bridge between existential risks and other possible risks. If we say that existential risks are infinitely more important than other risks, we put them out of scope of policymakers (as they can’t work with infinities). We could reach them if we show x-risks as extreme cases of smaller risks. It could be done for most risks (with AI and accelerator's catastrophes are notable exceptions).
Smaller catastrophes play complex role in estimating probability of x-risks. A chain of smaller catastrophes may result in extinction, but one small catastrophe could postpone bigger risks (but it is not good solution). The following table presents different levels of global catastrophes depending of their size. Numbers are mostly arbitrary and are more like placeholders for future updates.
http://immortality-roadmap.com/degradlev.pdf