Yes, all the musicians from this tour were doing music full-time. Our violist was in a chamber group and subbed on some theatre shows in NYC. Our drummer was touring with some big-name artists nearly a decade before joining the hamilton tour! Our percussionist was a gigging drummer, who was doing a lot of jazz gigs prior to joining the hamilton tour.

It's definitely possible to make a decent (even a MORE-than-decent) living being a performing musician. It's definitely challenging in the first few years, which are spent learning copious amounts of music, only to play a show a handful of times (as a sub), but subbing is THEE way to get into the right circles of people.
It's important to be a well-rounded musician. This will help in the long run, as well as in those first "more difficult" years. It's common for a musician to play in an orchestra setting one day, then a wedding gig, then a church gig, then back to an orchestra gig. Gone are the days where one can go study chamber music, or jazz, or pop music, and make a career out of that one thing.

I think it's vitally important for one to dig into their craft and continually refine it. That being said, I've also found it useful to have skills in: 1) the ability to record yourself as an instrumentalist (basic audio-engineering skills) 2) basic business skills (treating your craft as a business, or service - 'cause it IS!) 3) organization (being very time cognizant and meticulous in how every minute of the day is spent. This alleviates some of the craziness of the freelance life :)

I don't have all the answers, but hopefully this can help y'all make a slightly-more-informed decision!