The wireless industry in the U.S. essentially belongs to four companies, but only T-Mobile can call itself the disruptor. It’s the sole carrier simultaneously adding subscribers quarter-after-quarter at a rapid pace, creating new value through customer-friendly incentives, and investing heavily in a network built for the future. All those things, however, come at a very steep cost. And that’s T-Mobile’s undiscussed question mark.

While its three major competitors have massive amounts of cash on-hand due to their presence in other businesses, T-Mobile is doing everything alone. The revenue generated by T-Mobile largely comes from you, the customer. Little is made elsewhere. It makes you wonder just how far T-Mobile can go before it needs to bail on being the people’s carrier.

Verizon now acts as a media company, producing content from Aol and Yahoo as it sells home services through the FiOS brand. Sprint may be like T-Mobile in its focus on the mobile industry, but parent company SoftBank can easily cover expenses. AT&T, meanwhile, acquired DirecTV in 2015 and a merger with Time Warner is in the works. What were once only carriers have become full-blown media companies. It’s the one way T-Mobile hasn’t adapted.

T-Mobile is bringing in more customers than anyone else. The carrier’s added at least 1 million new customers every quarter since 2013. That happens to coincide with the start of its Un-carrier movement, which overhauled the brand’s image to make T-Mobile look like the hero the wireless industry always needed. John Legere, with his leather jackets and foul language, barged in and rattled an industry that was known for ripping off customers.

Today there are around 70 million customers on the magenta-colored network. Most of them arrived in the last couple of years due to the deals, discounts, and freebies offered.

The movement’s first step was to eliminate two-year contracts. T-Mobile acted, the other carriers followed. Now, regardless of the carrier you belong to, you can only buy a mobile device on a payment plan or at its full retail price. Then T-Mobile removed boundaries for roaming around the world. It also gave free in-flight Wi-Fi to customers through Gogo and created a weekly promotion in which free products and services are handed out like candy. From T-Mobile, you can get a free Papa John’s pizza before boarding a flight and enjoying an hour of in-flight data on your free phone. Why? Because T-Mobile needs you. So, to keep you, T-Mobile creates flashy incentives.

Perhaps the most notable achievement in the movement is Carrier Freedom. In early 2014, T-Mobile began paying early termination fees for new customers joining from Verizon, Sprint, and AT&T. It’s become a standard practice in the wireless industry.

And now you can also get a free Netflix subscription just for being a customer on a family plan. Yes, the carrier is covering the $9.99 per month charge over a 100 million people pay normally pay. T-Mobile will be on the hook to spend millions every month so its customers can appreciate them for access to the popular video streaming service.

The biggest expense the company faces actually doesn’t come from its customers, though. In addition to shelling out big bucks to ensure its network serves a large capacity of users in 2017, T-Mobile is building for the next generation of wireless technology. After being outbid in countless auctions, T-Mobile finally won spectrum to lay the groundwork for a 5G network. The price? A whopping $8 billion. The 600MHz network is being tested in select markets, and it should be ready for consumers by the end of this year.

Building (or improving) a network doesn’t come cheap. Although the aforementioned auction was a bit more expensive than usual, even a small piece of spectrum can cost hundreds of millions of dollars. A carrier then has to implement it, which is also pricey.

Here’s the thing: T-Mobile isn’t fueling its actions in the wireless industry from non-wireless activities. All the carrier relies on is your monthly bill. Unlike Verizon or AT&T or Sprint, T-Mobile doesn’t have anywhere to turn to.

When subscriber growth slows or comes to a halt, John Legere is going to have a big problem on his hands.

What are the best routes to take next? That’s complicated given T-Mobile doesn’t have the money to acquire a company like Dish Network and get into television. Also, it’s struggling to figure out a merger with Sprint. So, as a company, T-Mobile is in this awkward spot where it has cash and a solid image but could use help. A media company like Comcast or Charter wanting to immerse itself in the wireless industry might take a look soon. Comcast currently operates Xfinity Mobile on Verizon’s network while Charter chatted with SoftBank over the summer about doing something with Sprint. Picking up T-Mobile gives them a new service on its own network.

No one wants to see T-Mobile’s Un-carrier movement come to an end. Too much went into it to get here. And, if it is Comcast or Charter scooping up the carrier, the years-long hard work to build a legacy will seem meaningless to consumers. T-Mobile has trust with its customers, something most carriers in the U.S. can only dream about. It just needs to discover the importance of diversifying or risk letting someone else control its future.

There’s going to be a time, likely in the next year or two, when T-Mobile simply can’t keep up and it’s forced to scale back or sell. That’ll be what defines T-Mobile and John Legere’s legacy.