Will never happen with carrier locked phones, the carriers have zero incentive to do this, they want you to buy another phone and renew your contract yearly or bi yearly same as apple. Its the same as Apple is doing just a different approach, the carriers stop updating OS forcing you to upgrade your hardware, apple gimps speed forcing you to upgrade hardware. Different approaches to same "Problem". Best bet is buying unlocked phone from manufacturer, such as the pixel if you arnt a techie.
At least with android you have other options to switch OS's and continue to get updates as long as you wish to use the hardware. i still have my old note 4 kicking around running a custom rom that gets updates monthly, its not my main phone but its still functional, i use it essentially as a remote to stream to my chromecast in my living room, as well as game on it. GF still has her note 3 running as well.
I leave my phone stock till the 1 year warranty is up then when factory OS updates end between the 1-2 year mark i root it and pick a well supported rom to run till i get a new phone in a few years, i generally keep my phones for 3-4 years as my main use device. I love having the freedom to be able to do this, I realize rooting can be scary to some, but you can pay professionals to do it for you if you are not a tech person, and its a small price to pay for 1-2 more years of usability from your device.