It wasn't just a bet on the wrong technology.

Sprint wanted to outsource their 4G network to Clearwire. Instead of investing tens of billions building a 4G network, they wanted someone else to do it for them. Of course, if this had been successful, Clearwire would have become one of the major carriers while Sprint became relegated to the status of an MVNO. Sprint didn't want this and their dealings with Clearwire were terrible.

Sprint kept starving Clearwire for funds. Sprint was paying Clearwire about $4/mo per 4G customer. This meant that Clearwire simply didn't have the money to expand their network.

Sprint not committing to a strategy and being too cheap to build their future is a pattern with the company since their 2005 merger with Nextel. Nextel's technology, iDEN, had no future. Sprint kept it around until July 2013. They were running a network that could barely offer 2G data speeds three years after competitors had launched LTE and 6 years after the iPhone launched. This isn't keeping a compatible network around like carriers will do with LTE for many years after 5G has launched - it was keeping a separate network around, often with vastly different coverage.

To put that in perspective, AT&T shut down a similar network (TDMA) in 2007/2008. The writing was on the wall that the future was data and more efficient technologies and AT&T had been building that and transitioned their customer base. Sprint, by contrast, announced in late 2010 (after Verizon had launched LTE) that their "Network Vision" would include spending lots of money on iDEN. That would be like a company today announcing that their vision for the future meant spending lots on 3G.

Over the years, Sprint became frustrated by Clearwire and tried to find someone else to build a 4G network for them - this time LightSquared. The only problem was that LightSquared's spectrum was meant for low-power satellite communication and is right next to the spectrum used for GPS. Experts and the FCC agreed that using that spectrum for high-powered 4G services would drown out GPS signals. But Sprint spent a long time happy with the thought that they still wouldn't have to build a 4G network.

And they didn't really have a ton of spectrum. Sprint had the least spectrum of the big-4 carriers, but did own half of Clearwire (who had a ton of spectrum). Sprint launched its LTE network before buying Clearwire and they had very little spectrum so they only had a 5+5MHz LTE network (while competitors used 10, 15, and 20MHz channels).

Could Sprint have bought Clearwire sooner? Probably not. They ended up buying Clearwire at a valuation of around $10B which is high given that Clearwire was basically facing bankruptcy.

Plus, that spectrum is hard to put to use. Verizon and AT&T became large carriers through the use of sub-1GHz spectrum that could travel far and cover the suburbs and rural areas. T-Mobile started really doing well as it bought 700MHz spectrum and started increasing its coverage and reliability. The 2.5GHz spectrum that Clearwire had would have significant disadvantages even compared to the 1.7/1.9GHz spectrum. That would mean having to invest in way more cell sites to get coverage. For T-Mobile, this is fine. They already have nearly double Sprint's cell site count, as a merged company they have the subscriber count to build more, and they also have enough sub-2GHz spectrum to support a strong base network in areas where the 2.5GHz spectrum won't reach. Sprint had none of that.

Of course, Sprint spent several years after the Clearwire purchase deciding to try an alternative way of building a network without spending lots of money. It didn't work. Neville Ray (T-Mobile's network guy) said on an investor call that if there were a way of building a great network without spending billions of dollars every year he'd love it, but he hasn't seen anyone make that happen.

It wasn't about a single mis-step on the choice of technology. It was 15 years of incompetence and WiMAX was one of the least terrible decisions.

Ultimately, WiMAX was probably done in by Qualcomm most of all. Qualcomm was hugely threatened by WiMAX which would mean way less in royalties for Qualcomm. Qualcomm got Apple to agree to not create a WiMAX phone in exchange for lower royalty rates (https://www.ibtimes.co.uk/mobile-wars-qualcomms-actions-may-...). I'm guessing Qualcomm pushed on Verizon and AT&T to adopt LTE in the US (as well as operators in other countries).

WiMAX certainly performs poorly against modern LTE, but modern LTE has seen a decade of sustained investment. Initial LTE rollouts weren't nearly the same as we see today. It's also important to remember that Clearwire's WiMAX rollout was poor, but Sprint's 2.5GHz LTE network didn't do much better. It was also a dud leaving customers with poor service and inconsistent coverage. The failure of Clearwire and its WiMAX network might be more a story of underspending and spectrum that is too hard to use in the United States (where the population isn't dense and there's an insane amount of local regulation around building anything).

It wasn't Sprint's technology bet.