There are still design updates for competitor phones. And since the iPhone 12 came out we have seen a completely redesigned: iMac, MacBook Air, and MacBook Pro. All are mature products. Sure we got new processors, and what is possible with the iMac now wasn't possible with Intel chips. I can't speak for everyone, but I'm not expecting a revolution with an iPhone design update. Just a new design language every couple of years, that's all. So I'm not expecting a huge leap in functionality or some crazy wow feature. They do okay with those things still as far as I am concerned. I have no problem with iPhone engineering. I have a problem with the stagnation in design.
Putting aside something like a totally new type of device (a foldable or something of that like) what exactly are you wanting them to change?
Really the only thing they can do is slightly change the shape in the materials, which they have done over and over and over again.
Rounded aluminum, rounded plastic, flat glass and stainless steel, flat glass and aluminum, rounded glass and rounded aluminum with plastic edges, curved aluminum glass, curved stainless steel and glass, oval camera bumps, square camera bumps, circle camera bumps, slightly curved titanium and glass…
I’m not sure what else you want them to do, there’s a limit to what can be done to a screen that fits in your pocket.
As for those other devices you mentioned…
iMac didn’t get a design update between 2012 and 2021. Eight years!
The MacBook Air didn’t get a design update between 2010 and 2018, another eight years.
Between 2010 and 2018, the iPhone went from the iPhone 4 to the XS and XR, it very much changed design quite a bit.