Foe every poster who loves "4 iPhones and enjoying their current SE, they are far more outnumbered by consumers who can't go back to that small screen size anymore. Maybe back in 2013, it was still acceptable. In 2016 after a few years being accustomed to "4.7 and "5.5, that isn't the case anymore for most consumers. Face it, Apple is catering to what is now becoming a niche market that is no greater than Windows Phone. Cult following won't save it. Don't expect much growth in it or a mass exodus from the larger displays anytime soon. Perhaps Huawei catching both Apple and Samsung in market share by 2021 isn't such a pipe dream after all...
The biggest concern isn't really about the SE lackluster sales or if it had a different design or whatever. Apple could have changed it up a little outside and it still wouldn't sell as well since most buyers aren't going back to small screens anytime soon and not for $400-$500 bucks with lesser productivity and a worse viewing experience for media. The biggest concern is that maybe the iPhone 7 series will have lackluster sales too and Apple may have peaked already with iPhone sales. Whoever wanted an iPhone at whatever size, has one now. The signs of a decline is imminent now.
But I think all smartphones especially in the premium segment will start to struggle now. Phones have become homogenized. Your phone at whatever price range looks and acts the same like everyone else's with subtle differences in real world performance. The bubble got bigger and is ready to burst. Even the most dense Apple fanboy has to admit nobody stays on top forever and ever. What goes up must certainly go down too. To think Apple will have record sales every single year for the next decade or more seems unrealistic. It would be like expecting the New York Yankees and Los Angeles Lakers to win a championship every year.
Most people will hold onto their phones longer than before. The peak and luster of having the newest phone is on the decline. Again, the biggest worry isn't the lackluster sales of the SE or the same, lazy design it has. A design is such a superficial answer that will grow tired very quickly. Some of us expected it that way since smaller devices aren't going to make a comeback anytime soon no matter what it looks like. People need to see the BIGGER PICTURE and isn't on lackluster sales from a "4 iPhone. The biggest worry is iPhone sales ready to go on a decline in this oversaturated market? A proud history won't save them.
The decline of iPad sales started last year. The decline of iPhone sales could be happening this year. You shine and then you decline... Ask Motorola, Nokia, and BlackBerry about that.
While I agree with your assessment about why Motorola, Nokia and Blackberry have been all but wiped out, Apple has one thing going for it, which is going to be hard to make a dent in : its brand image and customer goodwill : we didn't buy Nokia, Blackberry or Motorola with the same passion and expectations as buying an iPhone. Looking back couple of decades for example, we also didn't just buy Nokia because of how much we liked the brand or its products : there just weren't enough manufacturers to choose from. Hence Nokia built its erstwhile empire and promptly allowed itself to rest on its laurels which led to its eventual demise.
Cut to the current day and we have a complete range of Android and iOS products and we still buy Apple products by choice even though we know we are paying a premium for a premium product. Apple will do anything and everything to ensure that we remain 'loyal' to their products which is why they go out of their way to offer generous return policies, product warranties/replacements and generally excellent customer service.
At the end of the day this is what constitutes their brand power and image and they will protect and grow it at any cost : it is one of the few things that differentiate the increasingly saturated smartphone market : You can buy a Samsung/LG/HTC but will you get the same level of after sales service or support ? Unlikely.
Cheers !