I'm not quite 40 yet (36), but part of what I've done is the specialization mentioned in other posts here (in my case, distributed systems with a focus on communications tech), and I've also cultivated my "product sense" to the point where (as an individual contributor) I'll often define and design a product and how customers will use it, in addition to doing the actual implementation. In that sense I have a little bit of breadth; I'm not "just" an engineer, I can also address the customer needs that lead to building a product, and then later refining it.

Understanding customer needs and translating that into product definitions is something that will likely never go out of demand, and is needed in industries outside tech. And if demand for distsys goes out of style I'll just learn something else. I've already kind of done that, having cut my teeth on embedded systems, followed by a short stint in mobile before getting to where I am now.

Judging by what I see around me, I don't see the strategy being any less effective in 10 or more years.