I get by just fine with 16GB as long as I have unlimited data. I stream all of my music and store all my pictures and videos in the cloud. I use Hangouts for texting, so only recent messages stay stored on my devices and don't take up multiple GBs like Messages used to.
Since I knew I wouldn't be using my iPhone SE much with an unlimited data line, I got the 64GB, but my iPhone 5s before it and my current iPad Air 2 and iPad mini 4 are both cellular 16GB devices and they're fine for me.
My point exactly. Everyone is different.
I live where internet and phone data are very unreliable and slow so it's either have it on your phone or do without.