I owned a Kuro 151FD that I regretfully sold off way too early to help cover moving expenses. I replaced it with a Panasonic V10 and eventually sold that off for a Panasonic VT60. I sold that in 2016 when I thought I’d be ready for OLED, only to find that uniformity, upscaling, and shadow detail were not up to par. It wasn’t until the Sony A9G that I finally found an OLED that qualifies as a “keeper” - which I attribute to a combination of improvements in the previously mentioned areas, as well as just sheer luck with the panel lottery.
As others have said, there’s a certain organic “look” to plasmas that is very hard to describe. It reminds me of the old CRTs that I grew up with. When I go to restaurants that still have plasmas on their walls, you can immediately recognize them just simply by the way the picture looks. Fluid motion, natural colors, and just a very nice, organic looking image.
Unable to do side-by-side comparisons, what I miss most about my plasmas was this organic look, as well as better handling of poor-quality sources. The A9G is no slouch when it comes to watching compressed content, but I still think the plasmas handled poor content a little better in terms of producing a “clean” image. I can't speak on the C9, but I'd imagine it's fairly similar.
I also think that side-viewing was still better on the plasmas, which most would agree with. Although OLEDs are far better than LCDs, I still see a bit of color shifting when I move off-axis - which was never noticeable on my plasmas.
What I don’t miss, however, was the image retention issues with my VT60. The A9G is much, much more resistant in this regard. Any IR I’ve spotted on the OLED, which I usually only see when immediately switching to a solid color screen, melts away within seconds. The Kuro was pretty resistant in this regard, but I’d have logos that would stick around for a long, long time on the Panasonics. I was never fully comfortable with logos for extended periods of time on my VT60.
Shadow detail might still be superior on plasma, although I haven’t had my A9G professionally calibrated yet. This is an area that OLED has improved since its earlier days.
But as other people have suggested, if you’re only going to watch compressed cable, streaming 1080p/720p and watching BD discs, I think the plasmas would do fine for a few more years. But if you’re going to make the jump to 4K/HDR content, then I feel the OLED benefits outweigh the few advantages that plasma still has.
Once you get used to watching content on an OLED, it's very hard to go back to any technology (whether it be LCD or plasma) that still has a "glowing" black in a dark room.