The only real tech mis-step that Sprint made was going with WiMax. But that was relatively short-lived. Sprint was an early adopter of Qualcomm's CDMA and it is still running it because the tech is solid. The Nextel purchase wasn't really for the tech (no one believed iDEN was the future), it was for the frequency allocations. Sprint was exclusively on the PCS frequencies before it purchased Nextel. Putting their CDMA on the lower 800 band was a good idea. Verizon excelled in rural areas using the same combination. Before Sprint had 800MHz bands, it was pretty much unusable outside of metro areas (usually roamed to Verizon).
So Sprint's evolution was CMDA 1x->CDMA 3G->"WiMax 4G"->(buy Nextel... eventually put CMDA on it)->LTE. Verizon was simpler as it used CMDA and then jumped right onto LTE.
AT&T had a much harder time switching out the various incompatible technologies on the towers over the years. AMPS, GSM (EDGE), UTMS, LTE, 5G(?). What's the difference? Clearly tech is not the problem. AT&T was able to juggle the band and tech. Good tech doesn't mean that a company will do well.
I've been on Sprint for a long time. Back in the day, Verizon was really restrictive when it came to what phones they allowed on their network. Verizon initially didn't like the Palm phones because a user could install whatever app they chose. This is why I initially went with Sprint. I live in a metro area and Sprint was friendly to tech friendly phones[1]. Verizon wanted their own ring tone and app stores. Sprint on the other hand embraced Palm phones. The big breakthrough was when Apple partnered with AT&T with the launch of the iPhone. That changed everything.
[1] The other option was AT&T's GSM and buying an imported Nokia Symbian phone. My brother went this route, but the upfront costs were higher.