I don't know if it has been mentioned or not, but the "fall" of the ST classes is dependent on area maybe. I know a ton of people that run it around here (especially STX/STU in WRXs and STis). That said, one reason that alot of the turbo cars no longer run ST is the restrictions on increased boost/tuning. Alot of turbo cars now have TERRIBLE tunes from the factory (if you'd like to see the a/f on my 09 wrx, you'll agree). So many (like me) bump to stage 1 just to get better streetability of the car. Unfortunately, this isn't ST legal, so it's up to SM or SP for me, unless I revert to stock tune for events. And again, the stock tune suck so bad I don't know if I want to do that. Thank the gov't for emissions regulations, and SCCA for not.
Also the large amount of other clubs doing autocross takes away from SCCA. I didn't run SCCA this year, but instead ran with the local BMW club, which had 4 classes for non-BMW cars (based on displacement/rotary/turbo/r-comp combinations). That let me build my car the way I want to - rather than try to stick to SCCA regs, which in some cases are pretty out of touch with what people do for mods these days.
Example: Subaru people love to get the 4-point underbrace. It really tightens the car up and makes it a better daily driver (especially over lousy roads). But it's not legal for ST since it's a crossbrace. SCCA acts like it is some kind of uber-high-performance mod that will give unfair advantage for autocross....when on some cars it doesn't help autocross at all - it's just good for street driving.
Not gonna go on about SCCA much more, but the increasingly complex rules turn off people who spend 99% of their time DRIVING their cars and not solo-ing them.
Hell, by the letter of the rules, I don't think you can even debadge a stock-class car, lol.....as if that changes its performance or something.
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As to "sport compact scene" in general - I think it's just as big as it ever was - and perhaps even more supported by large sponsors as in years past for that matter. But as many of "us" get older, we probably aren't as involved with it, so it may just be a perception that it is "fading" when in reality it isn't?
Either way, I was never really into the "scene" anyhow (aside from autocross, but I drove a Triumph, a Maxima, and now a WRX....none of which were traditionally part of "the scene.").....nor do I care if big "import tuner meets" happen or not...