First, Classic Rock stations are not giving up on the 70s and even some 60s music. But an occasional 90s song isn't a major shake up either. WAXQ in NYC plays 90s songs a few times a day... Nirvana, Red Hot Chili Peppers and a few others. I'm sure they test the 90s songs to be sure they are acceptable to the audience.
Second, there's a reason why Classic Rock can go back to the 60s and 70s when AC, Classic Hits and other formats cannot. It's because as we were growing up, Rock stations played more library material than other formats. Top 40, Country and Urban formats rarely played music more than a few years old. But Album Rock/Progressive Rock stations played music recorded decades earlier. Maybe an average hour would only include four or five titles released that year, a clear contrast to other formats that played almost all current material. Even if you started listening to a Rock station in the 90s, you were hearing plenty of Beatles, The Stones, The Who, Creedence, recorded before you were born... or at least before you started kindergarten.
This is the reason why Classic Rock does well, not just with Men 25-54 but even Men 18-49 and in some cases, respectable in the Men 18-34 demo. So for those who are proclaiming that Classic Rock is about to abandon the 60s and 70s, it isn't true. Maybe someday, but not for now.