You most certaintly did not leave for the same reason. AT&T is a special level of shitty with their device whitelisting. Verizon and T-Mobile do not whitelist. Verizon shut down their old CDMA network and you needed new devices.
3G WCDMA (which is what topic is discussing) was the last circuit centric standard, it actually sucked for footprint and was way more complex on the carrier end to deploy. It was so bad that when the iphone 3GS launched signal storming kept crashing out AT&T's 3G network.
Haven't missed using 3G on T-Mobile but they actually still keep the 2G signal alive in their guard bands which is fun for running retro devices.
> Even today I keep the 5G functionality of my phone turned off because 4G gives a consistently more reliable connection and faster data rates.
Verizon and T-Mobile have enough resources on 5G now that breaking gigabit speeds is pretty common, without mmwave. I've run my primary net on wireless for most of the decade and it's improved every year as new bands get deployed. First year of 5G was not great, now it's fast and most of the spectrum is on it.
However, the vonr/volte 4G/5G sip profile situation is really shit even without asshole operators doing whitelisting, it's a problem for emergency calls and probably one of the reasons t-mobile kept their 2G on air still, just as a CYA.