I've a 4G feature phone, and even when only its 2G modem is enabled, it doesn't last as long as the prior 2G phone did. When the 4G modem is enabled (which also uselessly enables its 3G capabilities), it discharges a lot faster.
The issue isn't the size of the battery, it's that 4G and 5G devices drain batteries on standby at a much higher rate, whether data is on or off. This is why I turn 5G off on my new smartphone. The increased speed doesn't justify the increased energy consumption for me.
And thanks to smartphones, people are used to charging their phone every night, so it doesn't feel like a hindrance like it used to.
Incidentally, 3G capabilities would only be used when the phone's searching for networks or connected to a 3G network (I think), which are becoming very rare nowadays.
2G due to its lower information density is also easier to receive and transmit generally allowing significantly lower density of towers and longer reliable range. In largely rural areas there aren't enough customers for companies to invest in towers for the same level of 4G coverage.
It was actually making calls which cut in to the battery, but still it could last 7-10 days with the pattern of usage I had.
That is really what is needed for simple "feature phones"
The thing that stops developed world from switching off are long term contracts of IoTs. But last I heard most of those will be ended by 2030.
4G and 5G also opened up more frequencies, some higher and some lower; lower frequencies generally travel further so assuming spectrum is available to carriers, I would expect most towers, at least in areas with coverage problems, to start using new frequencies when they get upgraded to 4G.
Certainly while carriers are building out, there will be pockets of coverage where older generation is better, but personally, I found I got better coverage once I was able to use 5G. I was in a good coverage area when I moved to 4G, so I remember that less for increased coverage and more for increased quality of service.
BS reason? Nicaragua keeps 2g and will "soon" implement 5g.
Edit: Nicaragua