No, in the context of real luxury goods, that's still not significant: Compared to a thirty-year-old purse, a seventy-year-old watch, or a three-hundred-year-old piece of jewellery, there is no difference between a five- or an eight-year lifespan of a phone: They're both "flimsy shit; here today, gone tomorrow" in that perspective.
Also, a phone will hardly ever be _the_ "second largest personal investment many lower-income people make in any object" -- given how ephemeral the phone's service life is, if those people live past thirty or so, it'll have to be one of several successive such investments. Which is precisely the point: If phones weren't such flimsy pieces of crap, it might be. (Oops, sorry, seems I'm full-on channelling Pirsig here...)
But no, BTW, actually I don't see all that much of a difference between a 15-yr mortgage and a 24-yr one. If you get to live your Biblically allotted three-score-and-ten (or more), then they're both "a substantial chunk" of your life. Depending on how a lot of other shit goes -- do you win the lottery, get cancer, have one kid or six...? -- one could be better, or the other. The difference in runtime isn't all that significant.
Same thing, only in reverse, with your phone lifetimes: Five years isn't all that different from eight, in that both are rather equally IN-significant chunks of your life.