[quote=@Trigani]
I understood it. The process of making a majority of what we use today would be too difficult to create due to the lack of certain resources.
[/quote]

And lack of manpower.

Over generations too the knowledge might be put on the wayside and then forgotten as people come to focus on what's relevant to their new life-style. People living in the Indian Federation out in New Mexico are more likely to pass on Native American survival techniques than computer engineering in much the same way people in the Plains States will pass up on learning modern engineering in favor of learning how to track, hunt, and probably tend to returning Buffalo herds.

I don't know if three or four is enough per-say, but there'll be a process of that happening certainly and we might be in the middle of that still. It's like how a language dies, you forget it in favor of something you will use for sure because it's relevant to your survival, your family's, and your communities. Knowing how to program a computer isn't going to help anyone when they're suffering from malnurishment.