I am using an android phone. Here is what would make me change my device:
1. I have enough money to afford an iPhone
2. The screen in my android phone is a little damaged due to water ingress. When it gets out of hand, I should think about changing it.
3. When there are any new advances in technology which I cant resist at all.
I make such choices because I use my phone only to call and to play one of my favourite game apart from ordering food and grocery max.