Question: My setting is what I would call "post-post-apocalyptic", meaning that our civilization doesn't exist anymore, but the collapse has happened in a past distant enough that people aren't really dealing with the direct fallouts of it anymore, or at least those aren't the story's focus. I need this setting to :

* Be technologically medieval, save for a few things like firearms – there are story-specific exceptions, but the general population in most of the world doesn't have even the notion of electricity generation ;
* Have very little to no collective memory of the old world and its history, and a very blurry/distorted memory of the collapse.

My issue is that both of those require a radical, virtually permanent knowledge loss, one that is complicated to make happen in our modern, interconnected world. For this reason, my question isn't about what exact type of collapse I should go for, but whether my focus should be "as few people left as possible without Humanity being doomed to extinction."

The idea is that going below certain figures would lead to a statistical lack of specialized individuals to operate/fix/produce modern technological items and act as teachers ; not enough people to spare from essential survival tasks and educate efficiently, let alone send to collect the knowledge still lying around ; large portions of that knowledge would gradually be lost to time as every electronic storage becomes permanently nonfunctional and books are left on their unprotected shelves to endure whatever comes their way for God knows how long. One can also assume that literacy rates would spiral down to abysmal levels, further hindering future generations' ability to retrieve any knowledge from those books on their own.

Until population recovery happens in any significant way (which I'm able to make as long as it needs to be), all those problems would only worsen until common and even "higher" knowledge stabilize around levels that we left eons ago.

So, to summarize :

* Preservation
* Transmission
* Recovery

Do you consider this a plausible way to reach the outcome that I described, or am I underestimating the resilience of our civilization when it comes to either of those three points?

Answer: 
This is not only possible, it's happened.

The last Mound-Builder culture of the Mississippi River valley collapsed after the introduction of European epidemic diseases, causing the population to decline to the point it could not maintain the chiefdoms it had consisted of. Some tribes managed to maintain legends that connect them to their ancestors, but others even lost the knowledge that the mounds they lived next to were built by human beings.

Likewise, the Bronze Age collapse meant the loss of the knowledge of how to read Linear B and Linear A -- indeed, only Linear B has been deciphered now.

And these were cultures with a lot less specialization and long-distance interdependences. It would certainly be possible for modern society.
"Too few people left" is indeed a credible explanation. Small groups of surviving people struggling every day cannot pertain knowledge. A reduction in numbers alone is not the point, 1 million people maintaining a city state can save a lot of knowledge, the same number of people scattered in small tribal groups all around the world can't.

There may be additional factors like deliberate destruction of knowledge and technology: After the catastrophic events, some prophets arise and found a new religion and demand to destroy the evil writings of the past that led to the current desaster. Such events have taken place in history in the late Roman Empire after the transition to Christianity, or at the introduction of Islam in the Near East region.