Granted, this is one person's opinion (and I'm not "picking on" that one person) but I really want to offer another perspective. By definition of the format itself, Classic Hits stations can't "remain '60s, '70s, and '80s, period." At some point, the '60s will have to be deleted and '90s will have to be added. Otherwise, the Classic Hits station turns into a Nostalgia station...and you see how many of those are alive and kickin' butt.
I would venture that '90s music feels modern because we were around for it...and my how the time does fly! But in 1990, Classic Hits stations were playing stuff from the '70s and probably stretching as far back as the late '50s! I'm sure that to listeners who were then the age we are now, the disco sounded quite modern! But station owners back then couldn't have played only stuff from the '50s and '60s any more than they could stick with only '70s and '80s, "period" nowadays.
There are indeed plenty of '70s and '80s songs to play. And a lot of them are toast bordering on charcoal. There's really no way around it: At some point, the '90s have to be added. Here's the thing though: Just like CHR exploded into a million different directions during the '90s, Classic Hits stations are going to have to follow suit now. And to an extent, we've seen that happen because, indeed, you cannot play Biggie and Nirvana on the same station. That's why we're increasingly seeing things like '90s Alt stations and Throwback Jams stations (it's also why Jammin' Gold was a thing for a while). Straight-up Classic Hits stations will add '90s but it will be the safest, purest pop stuff from the '90s. It's gonna happen. It has to.