I like to fly FS9 a lot more than on my older desktop than I do with FSX FSXSE and P3D installed on my newer (sarcasm, lol) pc.
When I was buying lots of payware planes way back in the day many that I did buy I did when a product was compatible with both FS9 and FSX...much like we see is the standard now with FSX and P3D payware -and freeware planes- nowadays.
Or maybe it's because flying FS9 on my older desktop somehow feels more nostalgic with no hiccups whatsoever even with them old payware goodies. On some days I just don't want to deal with the FSX/FSXSE/P3D stutters on my other pc flying hi-res payware planes...that's never been an issue with FS2004.
To the original poster asking the question asking how many of us simmers still use the old but trusted FS2004/FS9? I say 'thank you' for asking...and that question like FS9 will never go out of style imo.