Answering the original Q: I'd use the money to pay for travel and the other expenses of doing photography—I have two lifetimes worth of gear in the closet already. 😎
G
If we stop long enough to take stock of the equipment we already have - I could put all mine together and open a secondhand photo shop. I've been steadily downsizing since I retired in 2012, with a mega sale every few years to clear the cameras and lenses I no longer use and don't intend to. Yet the gear still grows and grows, as if it multiplies by itself in the night...
Later this year my collection of Rollei TLRs will go. I bought them all new from 1966 and I've treasured them (and used them), but in this all-digi age and given my own age they no longer suit. My Nikon DSLRs and now and then a Nikkormat or Contax G film camera, but all the more so the sad reality that film and darkroom supplies in Australia are now far too expensive for me.
Ditto my Leica iif kit (a iig, four lenses, a small box of accessories, all Leitz branded of course) which I put together after I bought the camera from a friend's deceased estate and a kind neighbor gifted me his long-unused collapsible Summicron 50/2.0. I used it for a few years as a now and then shooter but it now sits in my camera cabinet and I reckon it's time for it to go to someone who will not only cherish it but use it. Ditto the Rolleis, the multi-exposure kits, filters and other F&H bits alone which may well pay for a few of my airfares and hotel expenses in Asia.
Since 2012 I've offloaded Hasselblads, my film Nikons, most of my Nikkormats, various brand lenses and such off-bits as a mint Fuji GA645i. I bought it all at reasonable prices so I've made a little profit, and I've used all that gear but age is catching up, and I must start thinking about clearing the home nest before I'm too old, too tired, or too disinterested to do all that work.
Many of us are in this situation, I think. In our 70s downsizing is the way. So that fifteen grand would be useful to me but more so as an enhancement to my photography (= travel) and not for new gear - unless I decided to jettison my hoard of Nikon DSLRs and go with one kit. Now if a good Leica Q2 or Q3 were to turn up with my Melbourne secondhand dealer...