My experience is different ; Somehow I befriended a bunch of professional opera and orchestra singers in my mid 20s. It is now 20 years later and they absolutely all have side gigs.

We need to distinguish "professional opera singer" and "professional orchestra player" vs Pavarotti though :). Just like there are hundreds and thousands of "professional singers" and one Jennifer Lopez (though one could argue she's a perfect example of side gigs:).

My musician friends teach, do social networks, do all kinds of gigs, and are usually "multi barely employed". Not a single one of them, and two are fairly successful, are "full time employed" in the same sense a corporate professional is :-/.