I think you are talking about the US. In Europe (at least France & Germany) most professional classical musicians are doing it exclusively (we might be an outli

I think you are talking about the US. In Europe (at least France & Germany) most professional classical musicians are doing it exclusively (we might be an outlier but it's the norm here), competition is fierce though but at that level it's a full time job. In France we have prestigious ensembles (Orchestre de Radio France, Opéra de Paris, Orchestre de Paris), roughly 20+ subsidised regional orchestras and opera houses, military and police ensembles. That's for full-time permanent job. Underneath that there's the intermittence regime (roughly 100k people) freelance entertainment professional. Between gigs, the government provides a special unemployment insurance program to maintain a steady income. Germany is the same story, on an even bigger scale (100+ state-funded ensembles) but no intermittence equivalent. But even in the US, are you sure that musicians from the big 5 need side gigs? It seems that the salary in those orchestra are in the $100k-$200k range annually. Enough to live even in the most expensive cities, right?