I would still love to "fail" the way Apples does, but I think their recent iOS software quality issue and the schism created by offering both iPhone 8 AND X in the same quarter ultimately impacted there sales. Had, perhaps, Apple focused only on the X release, and then offered the iPhone 8 as a mid-cycle "value" phone, they might have seen much higher sales figures.
I mean sure, Apple "fails" in this market better then most other companies succeed, but if you look at Apple specifically compared to their past performance, as an investor you should be worried about last year. Apple made decisions that impacted its growth, period, both by releasing unpolished OS software AND by trying to dump a bunch of different, but not very different, phones on a market at the same time. Both speak to the quality of leadership at Apple.
Apple can usually bank on whatever quarter they release their iPhone as being their best quarter for that year, the other 3 quarters are usually mediocre by their standards, and even by their competition. If Apple screws up a quarter, even in a way that still has them emerge as the top phone maker in that quarter, it should be concerning to investors. The other 3 quarters are not going to make up for the lower then expected iPhone 8 and X sales, historically they never do. So in the end 2017 was a miss for Apple, period. Ultimately X will not grow the brand as much as Apple hoped for.
Whether this is a one time snag, or an emerging trend is yet to be seen, but at the end of the day Apple is ONLY worth its market cap if investors trust it can deliver growth on every iPhone release, otherwise it will slip to becoming an also-ran company that might have very good sustained profits but not the growth like what Apple has been used to over the last 10 years. One trillion in market cap means nothing because it can evaporate very quickly if Apple continues to lag on software AND hardware quality issues like iOS 11 and batterygate.
Regardless of how well Apple did compared to the competition, Apple is in recovery mode, or at least should be, based on their own performance history. If consumer confidence, and investor confidence, starts to chip away from their brand even a company as big as Apple can lose a lot of ground quickly.