I am typing this from a phone that I am already considering replacing for this very same reason although it does everything well and I looks brand new (rubber case, screen protection, etc.).
I understand phones are harder to upgrade because space is very limited but the e-waste we're generating (and the money impact) seems something that needs to be addressed.
My point is that with reasonable apps old phones work just fine. Just don't install crap apps or facebook or something like that, stick to well working apps. Use a phone as a phone, not as your universal computing interface and you should be OK for a long time.
Sounds limiting. Nowadays there are plenty of phones that can handle reasonable workloads at a reasonable price. Especially if you don't care about the camera.
I am aware more and more people only use smart phones and don't own a general computer any longer, but that is a really sad development. A smart phone will usually not encourage experimentation and getting into general computing. It is a dumbed down device.
Yes, one should be able to use a phone for computing, but generally that's not available out of the box and it takes effort to make it somewhat OK to use it for computing.
I used a motorola moto g (the one with 1 GB of RAM!) from 2015 until last year.
I imagine if this is something that lots of people want, it will result in more alternatives, but also raised prices. So it might not save money, but it might well result in less waste.