'Back in the day' when I was doing General Business (weddings, socials, etc.) in NYC if you weren't union the union rep would pull all the waitstaff, bartenders, etc. out of the hall or club unless the band stopped playing. The union also dictated the number of musicians needed to play certain sized rooms. This was a help to musicians because it got more guys working. However, Local 802 in NY lost a court case brought by some guy with 'house' contracts and the union could no longer enforce the minimums, or the union membership requirement. As that protection was about the only thing the dues got you, 'part time' musicians rapidly dropped their membership. .... and then 'along came DJs' .... and the rest as they say is history. I would say that the full time guys, playing theater etc. still are union members, as mentioned by 'Uncle Dave'.
Another change I've seen is that 'in the old days' you could play what was popular at the time, but do so in your own style, and people enjoyed it. In fact, groups took pride in developing their style, and their following. Nowadays, if it doesn't sound like the record, people don't want to hear it. So, in places like NYC, you have the option of hiring a DJ to spin records (at no cheap price), or pay 5, 6, 7M dollars for a 6-10 piece group that WILL sound like the record... especially with professional backing tracks, etc.
So now, living in Rhode Island, I keep content playing my kn6000 in lounges and supper clubs, and the occasional small party...no union, no hassle ....
t.
PS .... sorry if I digressed....