If you expect to find a problem, you will find a problem. Some also expect those problems to have a particular cause ("greed," "sloppiness," "death of Steve," etc.) - if and when they find that expected problem, they are already sure of its cause.
My mother used to say, "Some people are always looking for the hole in the doughnut." Others seek pleasure from their doughnuts and are happy to enjoy them for what they are. It's why some people manage 50 years of happiness with the same spouse, and others can't manage to last more than a few dates.
Around here, this human tendency finds its way into accusations of "Hater" and "Fanboy." It may be neither. It may just be an expression of a person's overall outlook on life - half-empty vs. half-full.
All products are refined over time. Every iPhone is an evolution from the previous. A flaw in the 7 may be fixed in the 7s, a flaw in the 7s may be fixed in the 8, and so on. Improvements can happen at any stage, as can unpopular changes. Apple has been using the "S" designation to identify the second year of a two-year cosmetic design cycle. Exterior appearance is identical or nearly identical, but internally there can be many changes. For marketing purposes, there's a benefit to bundling one or two marquee changes (eg. Face ID, OLED displays) with the biennial change to the cosmetic design language. However, regardless of year, the silicon and OS are constantly evolving. Display specs typically change annually, as do CPUs and cameras. If you tried to use an iPhone 7 display on an iPhone 7s, it wouldn't work. The changes might be subtle, but changes they are.
I think Apple recognizes that there are marketing/customer acceptance advantages to keeping the same external appearance for more than one year. People more readily recognize the product when they see it. People also want new features and improvements every year. They deliver those as well.
And to directly address the title of this thread, of course we're "test bunnies," if that's the way you want to view things. A manufacturer (or software developer, movie producer, musician...) can't be 100% sure of public acceptance until the public can experience it first-hand. They can dish out tried-and-true formulas, or take a risk of one sort or another. If they're good at what they do, they'll gauge the public's reaction and adjust their next product/production accordingly. If nothing ever changed, there would be no need for test bunnies. Since there's no such thing as perfection, the public is inevitably part of the process of striving for something that manages to come a bit closer to perfection.
