Stagnant implies it's not growing, not that it's not selling.
The global market size for mobile phones is still growing. If Apple's sales aren't growing at least in line with the size of the market then their market share is shrinking. Selling a lot in a stable market is great. Selling a lot in a growing market is ... less great.
Once your brand starts taking on the characteristics of a Veblen good, the rules start to work a bit differently. It hasn't, thus far, been a losing strategy for Mercedes Benz to not have a direct competitor to the Honda Civic.
I must have been talking about the premium phone market then, obviously.
A fairer way would be to state the iPhone sales have simply not grown.
Do you think Apple should be concerned about the lack of growth?
That said they must surely decline, the number of people who are going to continue to drop 1k+ on the new iPhone every few years is going to be limited by the resale value of last years phone, a market which (anecodtally) appears is droping precipitously.