T-Mobile also merged with MetroPCS after the failed AT&T merger which was key for their resurgence.

T-Mobile's turnaround had a bunch of facets.

1) Increased spectrum. In 2011, T-Mobile didn't have the spectrum to launch a competitive LTE network. AT&T had to divest spectrum to T-Mobile as part of the breakup and they gained spectrum through their merger with MetroPCS giving them 20MHz of new spectrum. 20MHz of new spectrum is an LTE network right there. In some ways, AT&T's decision to offer spectrum as part of the breakup fee might have pushed the FCC to deny the merger. "T-Mobile can't compete due to its spectrum position...which would improve if you denied this merger and might put them in a position to compete."

2) Increased customers. MetroPCS added 28% more customers, 26% more revenue, and 29% more EBITDA (earnings before interest, taxes, depreciation, and amortization). In an industry that relies a lot on scale, that's pretty important. It moved T-Mobile from a very distant 4th place where Sprint was 61% larger to a much closer 4th where Sprint was only 21% larger.

3) Finances. T-Mobile's finances were a lot better than Sprint's and Deutsche Telekom reduced T-Mobile USA's debt as part of the MetroPCS merger by $3.8B. That meant that the new T-Mobile had a solid footing to go forth.

Sprint, on the other hand, is getting crushed by debt. T-Mobile's net debt in 2013 (when they started making their comeback) was $15.3B over 44M customers or about $348 per customer. Sprint's debt is $32.9B over 54.5M customers or about $604 per customer. A lot of that debt is coming due over the next couple years. If Sprint is paying that debt and doesn't have the money to invest in 5G, it's hard to make a turnaround.

Sprint has $27.6B in debt maturing from 2019 through 2023 (5 years). That's going to make it hard for Sprint to invest in its network during critical years for 5G deployment. They'll be competing against a T-Mobile that can get loans at better rates and has about half the debt Sprint has on a per-customer basis. T-Mobile is profitable and has revenues about 32% higher than Sprint.

4) Timing. T-Mobile launched the Un-carrier while customers were still making up their minds about LTE and hit the ground running fast. They announced targets for LTE coverage and blew past them inspiring consumer confidence. They broke new ground on pricing and features offering a more consumer-friendly approach to wireless - no contracts, trading in phones for upgrades, free international roaming, paying off ETFs to get switchers, music streaming that didn't use data, video that didn't use data, and finally unlimited. Likewise, T-Mobile was going up against an AT&T and Verizon that were only two companies and had become very anti-consumer and very unfriendly.

Today, the timing isn't there for Sprint to do the same. AT&T and Verizon were broken by T-Mobile. They now offer unlimited and while they might not be the best deal, they make sure to remain competitive.

Likewise, Sprint would have to compete against three strong companies instead of just two. That's a much harder proposition. These are three companies with way more customers. T-Mobile is now 50% larger than Sprint. These are three companies with more revenue and profits to invest in their networks. These are three companies with more low-band spectrum to provide broad coverage. These are three companies that aren't under crushing debt.

5) Spectrum Mix. Ultimately, Sprint's assets are different from the other three companies. Low-band spectrum covers a larger area and provides broad, reliable coverage. The other three carriers have 3-5x more than Sprint. Mid-band spectrum was used to supplement that for greater capacity and the other carriers have about double what Sprint has. Then Sprint has a lot of high-band 2.5GHz spectrum that doesn't cover as much area. Clearwire and Sprint have been trying to use that spectrum for over a decade and it's not fruitless or anything, but it's unclear whether that spectrum will provide compelling mobile service that will truly compete.

Now, that spectrum could be quite useful for 5G. However, Sprint doesn't have the finances to go on a huge spending spree to add more cell sites (and they have far fewer than the other three) and mobile use still might need more low and mid-band spectrum than Sprint has to create a robust network that doesn't feel patchy.

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I think it's easy to say that Sprint could do what T-Mobile did. However, I think there are key differences. T-Mobile changed its possible future by adding 20MHz of new spectrum (increasing its total spectrum by 38%). The addition of MetroPCS customers meant that Sprint was only 21% larger while T-Mobile stands at 50% larger than Sprint today (and that gap is growing as T-Mobile adds millions of customers each year). T-Mobile's finances were simply so much better. They were barely profitable, but profitable with so much less debt. The timing was right to disrupt the market as AT&T and Verizon kept making increasingly consumer-hostile moves and jacked up prices a lot, but that isn't the market Sprint faces today. Finally, it might just be that Sprint's spectrum mix makes it hard to create a network that customers love.

It's certainly possible that Sprint could turn itself around, but T-Mobile was in a very different situation when it launched its comeback.