Even the best companies in the world have detractors. Tech giant Apple (AAPL -0.35%) is no different. The bears can point to the fact that the iPhone no longer generates the same amount of buzz as it did in its early days, or they can point out that its manufacturing has become highly dependent on a sometimes volatile Chinese market.

These issues could present long-term headwinds, especially for a company already worth $2.4 trillion. Despite all that, Apple's investment thesis remains strong. In fact, there are 2 billion reasons why the company remains a solid buy this year. Here's the rundown. 

Apple's massive installed base 

Apple's December quarter, the first quarter of its fiscal year 2023, was perhaps disappointing. The company's revenue decreased by 5.5% year over year to $117.2 billion, while its earnings per share also declined to $1.88, about 10.5% lower than the year-ago period. Although these results are far from ideal, it's important to put things in context. The world faced difficult economic challenges last year, including near 40-year-high inflation.

Unsurprisingly, some consumers chose to rein in spending on products that aren't essential, such as the newest iPhone. So Apple's first-quarter performance hardly indicates how things will go once the economy recovers. And on top of that, Apple announced one piece of good news in its latest quarterly update.

The tech giant highlighted that it now has an installed base of 2 billion active devices. That's an impressive number. But it is by no means done growing. Here's why. 

There's more where that came from

Apple's most popular product is the iPhone, but it has plenty of others, from tablets to computers to wearable devices. One of the keys to the company's success here is that its software allows these devices to interact. iPhone users can easily read and send messages on other devices and share documents and photos between their Apple hardware, and that's just the tip of the iceberg.

That's why it is hard for iPhone users to jump ship since it would mean transferring data to another operating system, which isn't easy. Apple has the highest satisfaction and brand loyalty score among the major smartphone brands, and it attracts many first-time users too. That is precisely what CFO Luca Maestri recently highlighted: "This continued growth in the installed base is due to extremely strong levels of customer satisfaction and loyalty and a high number of customers who are new to our products."

So we can expect the company's users to stay put, and that 2 billion number will even increase, especially as Apple adds new products to its arsenal. The company is developing Augmented Reality (AR) headsets that will be released this year. And with Apple's successful track record at adding its own tweaks to existing products, these new products could be a hit.

That's especially the case considering Apple's brand name alone attracts consumers' attention. That's why it is one of the most valuable in the world. AR has been growing in popularity. That will continue for the foreseeable future. According to some estimates, the market will clock in a compound annual growth rate of 40.9% through 2030.

Apple will have plenty of opportunities to get in on the act. 

High-margin monetization opportunities 

Apple has been monetizing its user base via its services segment for years. From Apple Pay to Apple Music, Apple TV+, Apple Fitness+, and more, there is no shortage of options for the tech giant to serve the needs of its customers, and it will continue to find new ways to do exactly that. Some of the company's existing growth avenues are highly promising.

Consider Apple's fintech ambitions through Apple Pay and a buy-now-pay-later service. The mobile wallet market is still relatively small; it was worth $6.2 billion in 2021, but it will expand rapidly in the coming years. There were 507 million Apple Pay users as of Dec. 2021. That's out of over 1 billion iPhone users worldwide. Apple makes money through its payment service by collecting a fee every time someone uses it to make a purchase.

So as the adoption of this feature increases and there are more transactions, Apple will generate more revenue. Mobile payment usage is highest among Gen Z, so there is a good chance it will become more standard, with younger generations making up an increasingly large share of the population. And that's just one of the many long-term opportunities available to Apple.

Further, the company's services unit boasts much higher margins than its hardware business. In its latest quarter, Apple's total gross margin was 43%. The company's products and services segments reported gross margins of 37% and 70.8%, respectively. Apple isn't done generating money from its hardware products. But it will make an increasing amount of money from its 2 billion installed base, which will positively impact its profits and margins.

There is no end in sight for the monetization of Apple's user base, and that's a great reason to buy shares of the tech giant this year.