This is probably true to some extent as well but I'd argue it also has at least as much to do with the upper-classmen from the last couple years just weren't athletic enough to get on the field early so it was only upperclassmen for a few years. Now that we have better athletes they are getting on the field earlier.Also, I don't want to beat this all to death because results is all I'm interested in, but a lot of the names you listed were more played out of necessity and lack of depth, not because they came in and simply 'got it' right away. Those guys had their struggles too. We're still not talking about an elite Blackshirt defense. Not yet anyway.
To try to swing this back on topic, that also gives hope that some of the young guys that are going to make up most of the #2 unit now will also be able to contribute quickly.