I think smaller markets like Tri Cities, Yakima, or comparable markets of about 300k usually have huge format holes. Tri Cities has no oldies/greatest hits station and no true soft rock station. Plus no real alternative station. It is a radio wasteland which is why most of my listening is now SXM. I am amazed these locals stay in business these days.
But if your format is country or top 40, the money pours in. I'd even say the same for most of the Spanish stations in town. Not counting the non-comm Christian stations in Spanish, there are about 7 listenable stations in Tri-Cities with (mostly) Regional Mexican, albeit there is the 104.9 with Spanish hits and upbeat music.
No classic hits station in Tri-Cities either, you are right. 92.1 Pendleton is the closest, with a listenable signal in most of the Cities. I noticed 100.9 KARY put in a good signal into south Kennewick last time I was there. The transmitter is quite a ways east of Yakima however, on the big ridge north of Sunnyside. The last classic hits station licensed to the Tri-Cities was 97.5, which has since flipped to Rhythmic and now Top 40.
As for niche format holes, we'll likely never get another station with those formats again, but I sure do miss the old Mega 99.3 Yakima when they were Rhythmic Oldies. It was one of the best things to happen to the Yakima FM dial since I moved into the market in late 2013. Nowhere else on the dial could I find a similar mix of music. It was fun listening to Barry White, Bobby Brown, Teena Marie, Pebbles, lesser-known Whitney Houston and Prince, Smokey Robinson, TLC, and Earth, Wind and Fire. Then they went to classic hits and lost me as a listener.
Tri-Cities' 102.3 used to be smooth jazz before flipping to Tri-Country in June 2019. In Yakima, 38 watts was too weak to reach me even with a weak signal. Thanks to that online stream, I enjoyed their format and it wasn't just the greatest Kenny G hits. There were lots of smooth vocals and instrumentals mixed in. Incognito, Walter Beasley, Richard Elliot, Randy Crawford, Chris Botti, Lindsey Webster...lots more.