Agreed. I've run AROSC schools and TTs for 15 years, and the organizers there could bore you for hours about getting folks out to the track. There was a period a few years ago where several events were canceled due to lack of signups. Club loses money either way in that circumstance.
AROSC has run co-events with NCRC, generally work pretty well as mostly compatible folks and rules. The thing with running Alfas with other groups though, is that most of the Alfas are quite slow (at least the four-cylinder cars are) compared to Lotuses, Porsches, Mustangs, M3s, Corvettes, etc. that you see at most open-track speed events. I've done some of them in my GTV and Super race car, and it's just not that fun, as you're getting passed all the time and trying to stay clean and out of faster folks' way. So I've stopped doing those. That was one of my main motivations to run with CSRG, to be on the track with compatible, interesting sports cars, but also ones that are roughly my speed.
AROSC events are open to anyone, typically my recollection is 25-30% Alfas nowadays, lots of BMWs, Corvettes, Porsches etc in the non-Alfa groups. If it were limited to Alfas only, wouldn't be enough to field an event.
I don't think there are any easy answers here or we'd have found them by now.
Andrew