Has he upgraded? Or talked about what he sees as an efficient phone upgrade cycle should be?
No idea what MMM has done, but it's been a point of discussion elsewhere on the forum.
IMO, at this point, running a phone past mainline OS support is questionable, running past the end of the security update window is just stupid unless you're going out of your way to harden the phone in terms of apps/interfaces/etc.
I'm on the iOS side of the fence, mostly because the phones work, work well, are repairable, and don't have the sort of screwy hardware faults Google devices seem to struggle with (getting one to last 2 years of daily use, for the last couple generations, has been iffy). I'm getting into my 4th year with an iPhone 6S, and I'm pretty sure I'll get another year of OS updates, if Apple's traditions are continued this fall.
My plan, when my phone falls out of support, is to buy a generation-or-two old device, used but in good shape, and run that until it's out of support, wash, rinse, repeat. I'll probably eBay my existing device, just because there's a market for out of OS support hardware that I don't understand but will make use of.
Alternately, if I'm feeling particularly curmudgeonly, I'll just start reducing privileges on my iOS device until it doesn't have anything of value on it - no email, no passwords, nothing. And strip it down to barely more than the core apps. I'm not sure how useful that is, but... I'm also not sure how much I'm going to be using it once it's out of OS support. It'd be a good way to use my phone even less, making it more useless! But I do like access to email on the go. On the other hand, there's remarkably little I do with my phone I can't do with a small paper notebook and pen kept in my pocket. So...
But I'd be really, really uncomfortable running far out of OS support on any device at this point.