There is the problem - code reviews are done by humans and so it is really easy to read some code and not think "wait this is new code it can't do that anymore". I read a lot of old code as part of my job and it often isn't worth the bother to update working code to new standards.

> it's a lot harder to justify avoiding new features than it is to justify avoiding old features

The problem is the opposite - people keep using old features when the new is better. I realize not everything new is better, but most of them are, yet people keep refusing to learn and use the new thing without good justification.