I see nothing wrong with playing three or four songs from the same decade so long as they don't share the same texture. All bro country or all country rap or a mix of the two would chase off most of the older listeners, but an all-'10s set including, say, FGL's "Cruise," Miranda Lambert's "Heart Like Mine," Luke Bryan's "I Don't Want This Night to End" and Kenny Chesney's "Celebrity" would contain enough variety to keep most of the listeners tuned in through the one song they're not crazy about because, after repeated exposure to the station, they'd sense that a song done in a style they like is likely to be played next.
I'm just a fan -- of country music and radio -- so obviously, if you're a professional, you are much more likely to know what does and doesn't work. I'm just throwing the idea out that decade doesn't matter in putting together a modern classic country playlist so long as there's a difference in styles.