There was recently some advice (on HN?) that if you really want to make money, you specialize in 2 things, not just 1, and not Jack-of-all-trades. So having a first career and then moving into a second one is one way to do that.
Last year we hired a couple junior programmers, even though we were only aiming for 1. The second programmer impressed us so much with her code samples that we were tempted to hire her as well. But what really cinched the deal was that she had uncommon business sense and knew enough to tell us about it. She has been the best junior programmer that I've ever worked with, and I have no doubt she'll be an amazing senior programmer some day.
I'm 50, been programming for >30 years, and never had a client ask me that either, but recently I've found myself asking that a lot of the time.
I am at that stage where I know that I cannot work as fast as I did 20 years ago, and that my endurance for all night programming sessions is not like it was. Add to that back and shoulder problems, and degraded eyesight from sitting in front of a monitor for years, and I am thinking about what happens to my clients and my work if I am forced to take a break for health reasons.
I've spent time talking to a lawyer (and some of my clients) about putting source code in escrow with them, and letting other family members know where they can find a list of critical logins for all the services I use in my day to day work. Call it a 'work will' if you like, but I want to ensure that if I am not able to do the work anymore, for whatever reason, I want my clients to be looked after.
Counter that with "what happens if you get diagnosed with cancer and the next week your on chemo" This happened to a young guy in our team at my last bit one place and to my project manger earlier in my carrear
One thing you'll likely have in your favour is knowledge of another domain to a degree that perhaps no other programmer has.
Even so you might have to go the start up route and then unless you have strong marketing skills you're unlikely to do well.
"Is it just you? What happens if you die?" is a question you'll get over and over.