The tech is 10 years out of date, but it's so ingrained in the customer sites that it's going to stay there. There are plenty of jobs like this out there in finance, public institutions (or big companies serving them), the military (military projects can go on for decades) and so on. I get job security and a decent pension building up, they get an experienced developer in return.
And in the meantime I can choose to look at shiny-new-thing without having to worry my career might depend on it.
People might think 38 is a bit early to "sell out", but our industry is definitely ageist. It's not easy for non-techs to see the difference between an experienced dev who wants £100k that knows technology x and a graduate who wants £fuck-all and knows technology x.
As an older developer, I think one advantage I do actually have is that I do know legacy technology. You don't want to become a developer that only knows that, but as long as I reasonably keep up with current technology and don't become a Grumpy Old Back In My Day type of old developer, that should add, not subtract. At my current job, there are actually some VB6 apps still in operation, so sometimes I am actually pulling up VB6 apps to fix or enhance to go along with the shiny new Angular 2 apps we're working on.
Yes I know that I don't have the energy to pursue the move-fast-break-things life of startup style companies, but not every company is like that.
There's always something new and cool to learn. It's pointless if you never use that knowledge.
I am an old geek - (34 now - i guess)... I still need to work the 12 hour days in order to keep up with my day job AND to keep up with new technologies and the literature via Pluralsight, Github / HN / Coursera etc.
This is what I really worry about in terms of my health and longevity: Being able to "stay on top" of the latest trends and shiney new things - and deciding what will matter in 3 years, and what is just another "fad".