You are right. There seems to be an irrationality in some people's views about Apple being "doomed" because it "only" sold 51 Million iPhones last quarter.
This is the first quarter in the 9 years that the iPhone has been for sale that Apple didn't have a "record-breaking" quarter... If that makes Apple doomed, then what is Samsung (the next largest seller of high-end smartphones) which has been having decreasing sales of Galaxy phones and profits each quarter for the past 2 years?
At last count, Apple makes 94% of the profits of all mobile phones sold worldwide. Is this the sign of "doom"? (Samsung makes about 5% of all profits, and many other companies are struggling to break even, or losing money and dropping out of the race)
SiliconValley.com recently reported that Apple ranks no. 1 by far in revenue, and is making 40% of the profits of the tech industry. Last year Apple raked in $234 billion in sales. Trailing far behind in second place was Alphabet (Google), at $74 billion.
Apple currently gets most of its revenue from iPhones, but in the past that was from the sale of iPods, and before that from the sale of Macs. Apple (unlike other companies that continue to flog the same products) is always adapting and changing its business model to grow. Over the past few years, the revenues from Apple's services have gone from a tiny portion to a huge source of income. In yesterday's financial report, the revenue from services has grown 20%, and is now worth more than the income from the sales of Macs and iPads combined! And it has been reported that Apple is developing additional new sources of income (cars, VR, etc.).
With these facts in mind, only an irrational person would have the misguided opinion that Apple is "doomed". It is an indication of reality avoidance.