Re: Feature Phone is functionally better, and better value
I'm not following some of your objections, and some others are easy to understand but I haven't experienced them.
"Feature phone works well with pay-as-you-go. Smartphone requires contract (£20 a month is cheapest I can get)"
Why does it require a contract and why that contract? I've had smartphones on no-contract plans. It worked fine. The restricted data meant that I chose to prevent most applications from using mobile data, leaving only a couple I wanted to keep, but otherwise, it was fine. Is your device locked? Otherwise, why can't you put in whatever SIM you want?
"Feature phone starts in seconds (useful for banking 2FA). Smartphone - takes long enough to make coffee."
I just don't experience that. Smartphones I've had are pretty fast to unlock and cold start in at most thirty seconds. The only way they would take longer is if I've completely discharged them. I wonder if there's something unusual about your phone.
On some other points, especially the size, I definitely agree with you. Manufacturers seem to agree that people want long and wide devices, and while they hire researchers so they probably have some data to back up this idea, I am not in that set. Finding something small enough is a challenge and one I'll have to deal with when mine breaks, which I hope will be years from now.