I don't think we're losing knowledge of how the Internet works, but we're almost certainly losing knowledge of why it was done that way. I remember Bob Braden saying (and I paraphrase):

"When we designed the early Internet, we had a huge blank space to work in, and we agonized over what the best way to do things would be. Ever since, people have been filling in all the other parts of that space."

This was 20 years ago, but he's probably even more correct today. Of course they didn't get everything right by a longshot, but we're definitely losing the rationale for why things were done the way they were. As a result, it's quite common to stumble into old problems that had been engineered around before.