Tim Cook's Apple is hot. But can it keep going?

NEW YORK (CNNMoney) -- Tim Cook has given his naysayers the ultimate comeback: His company just posted the most profitable quarter for any company -- ever.

Cook has heard it all: He's not suited to be CEO. He's just an operations guy. Apple needs another Steve Jobs, not a Steve Ballmer.

He's not hearing much of that anymore. Even Cook's biggest detractors have to admit that Apple's recent performance is astounding: -- 74.5 million iPhones -- 5.5 million Macs -- iTunes sales records -- $18 billion in profit, 30% revenue growth -- $178 billion in cash last quarter. Can Cook possibly keep that kind of growth going?

The case for Cook's Apple isn't a particularly difficult one to make (investor Carl Icahn has repeatedly called it a "no brainer").

Consumers are going bananas over Apple's new iPhones. With iPhone market share hovering in the teens in China, Apple has a lot more Chinese customers to sell to. And Cook said this week that he is "incredibly bullish" on the iPhone going forward, because only a small fraction of iPhone users have upgraded to the new iPhone 6 and iPhone 6 Plus smartphones.

Meanwhile, Apple is making it harder for existing iPhone users to defect to Google's Android, by making the iPhone more symbiotic with the Mac, iTunes, Apple Pay, Apple TV, and all the other Apple services and iGizmos.

But there are also reasons to believe that Cook faces a tougher road ahead.

Too much iPhone reliance. Apple is hugely reliant on the iPhone -- the iPhone accounts for 69% of Apple's revenue. That's a potential problem, because smartphone sales are slowing in most of the biggest iPhone markets.

In the United States and Western Europe about 70% of cell phone customers own a smartphone. Going forward, smartphone sales will likely only grow by about 5% to 10% a year, according to Colin Gillis, analyst at BGC partners.

iPad slowdown. iPad sales continued to slump, falling 18% to 21.4 million last quarter. Sales of Apple's tablet have shrunk for four straight quarters and in five of the past seven quarters.

Cook defended the iPad this week, saying that half of the iPad buyers this past quarter purchased one for the first time.

But even as the entire tablet market slumps, the iPad has fared even worse than its rivals. Apple now only commands 23% of the tablet market, down from 29% a year earlier.

What's next? Cook has said that Apple Watch, Apple Pay, corporate customers and TVs are the next big growth areas for Apple. But all of those come with giant question marks.

Apple is betting that people will want to spend $350 on an Apple Watch that gets less than a day's worth of battery life and doesn't do anything that your iPhone can't do. Smartwatches as a whole have not yet taken off (though neither had smartphones or tablets before Apple brought the iPhone and iPad to the market).