We all know that Apple (NASDAQ:AAPL), famed for its Mac, iPod, iPad, and iPhone lines, is up to something. The fivefold increase in research and development since 2009 is likely indicative of some pretty serious innovation going on within Apple's walls. Now, of course, a good chunk of that is likely a result of an aggressive buildup of its semiconductor teams (the A-series processors will get even better), and the continual improvements in iOS and OSX aren't going to come any more cheaply, but there's something more going on here -- and investors can profit handsomely from it.

The iWatch drama
Apple's "genius" hasn't exactly been a result of coming up with a radically different/new product category. Cell phones and, yes, even smartphones predated Apple's iPhone. Tablets and MP3 players, too, came before the iPad and iPod, respectively. Apple's contribution has been the development of highly refined, easy-to-use devices that the masses found useful. This will be the flavor of whatever new device categories Apple ultimately has in store -- very refined and far more useful variants of devices that many of us use today.

Of course, given how persistent the iWatch rumor has been in the press, many of Apple's competitors, including longtime rival Samsung (NASDAQOTH: SSNLF), have already announced/released products in a bid to beat Apple to the "next big thing." Unfortunately, given the clunky nature of the products released thus far, it's clear that Apple's competitors don't really get it. If Apple does an iWatch, investors should be confident that it will most likely be the most refined offering on the market at the time of launch. And if the category really does have a future (remember, digital watches used to be all the rage back in the day), Apple will be the one to really take the lead here.

But it's not just the iWatch
When the first iPhone came out, it was amazing, right? Well, until the 3GS hit the market. Of course, that one was "it," the pinnacle of phones. Right? Nope. Fast-forward to today, and you'll hear the same old chatter about how it can't get much better than the 5s (other than, of course, a larger screen for some users). It's going to prove bunk and the iPhone 6, 6s, 7, 7s, and so on will all be pretty sizable improvements over what came before it. Users with the iPhone 7s will wonder how they ever got on with the "antiquated" 5s. These arguments also apply beautifully to the iPad. 

And it's that innovation cycle -- particularly as Apple is in control of both the hardware and the software -- that will ultimately drive iPhone sales steadily higher. Will it be an explosive s-curve of growth? No, probably not. Will Apple be doing mid-high double-digit year-over-year growth rate on the top line? No, probably not. At some point, the party must end. But can Apple be a great long-term investment? You bet.

But you just said not to expect high growth rates!
Yes, you're probably not going to double your money in a year holding shares of Apple. After all, at a market capitalization just shy of $500 billion, it's tough to add another $500 billion in market cap. But can Apple reward shareholders with a nice and steadily growing dividend and share price? Of course, it can. As long as Apple continues to grow its revenues at a mid-single-digit annual clip (or better), and as long as Apple keeps its operating expense growth roughly in line with revenue growth and margins stable in the 37%-38% range, Apple should be just fine. Not an explosive growth name, but a stock that can serve as a good sleep-well-at-night core holding in a reasonably diversified portfolio. And, frankly, it's these kinds of "slow and steady" investments that offer some of the most compelling long-term risk/rewards on the market.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.