Well for starters, 5GB is about £45 a month for me and wifi isn't so widespread, so I can't really afford to peruse the cloud like that. I have everything backed up on a hard drive but what's the point of a smartphone if you can't pull out photos or videos on a dime?
Also luckily my music listening habits have changed drastically lately and I use Spotify all the time and normally just listen to personalized playlists instead of carrying around 30+GB of my music everywhere I go like I used to. Still, I've found I needed a handy base on hand if ever I find myself without internet access. Basically, I think 64GB without expandable storage should be the bare minimum because like me, not everyone has the luxury of access to the cloud 24/7.