I think this logic is flawed - just because something sells well initially doesn't mean it should stay around forever. Every iPhone sells well; it's no huge surprise that the 6 and 6 Plus continued that trend. I think the lineup would seem a bit complicated with a 4" iPhone, two 4.7" ones and two 5.5" ones.
If that did happen, it's exactly what got Apple into the mess it was in the 90s verging on bankruptcy. Obviously that's not going to happen any time soon, but it's not the simple lineups with one or two products in each category which made Apple into what it is today.
They have history of killing off popular products too - iPod mini in early 2000s, iPhone 5 a couple of years ago.