It isn't so much how much you use your phone, its all about how much you plan on carrying around with you. Now, for many there is a logistical need to lug around several dozen gigs worth of media... your signal might be hogwash for most of the time or you travel internationally so you spend a lot of time in dead zones with no data or cost-prohibitive data options, etc...
People leave way too much junk on their phones. Most everyone here.. admit it... if you go up and down your list of installed apps, there's a fairly long list of crap that you haven't touched in ages. The digital hoarder moniker is 100% true... I know I certainly am guilty of the practice. There's a few here just looking that fall into the category of "I might use it, but I haven't touched it since like two phones ago"... Google, of course, enables me because it'll gladly bring down all that useless goo when I reset my phone or get something new. Filling a 32G+ phone with apps takes a lot of effort (or an extreme lack thereof)
Now... if you are lucky enough to live in an area with good coverage.. between mobile data and good wifi... a 32G phone will be a lot more useful than you would think. Would you have to change your habits a bit? Sure, but I bet the adjustment would be a lot easier than you think. Put your 20G MP3 collection you drag around like a boat anchor and stuff it up into Google Play Music, they'll store up to 50,000 songs for free. Even if you don't have a Pixel, Google Photos is still an option... either the HQ 'unlimited to all' version or the 'only first 15G free' original quality one.
As for 4K.... I'm on the fence with that one. I can do 4K now, both me and my wife have 4K capable cameras on our phones. But neither of us have any 4K videos. I just don't see the use at the moment. First, my wife takes all our videos and then creates video compelations from them.. "Summer 2016", "Disney" etc... and she does so in 1080 because anything higher would be a senseless waste of resolution, storage space and processing cycles... we don't have 4K TVs. The 1080 videos we make now still look pretty damn good on or large TV. 4K goes under the law of diminishing returns.
I have about 42G free on my 64G 6P.... 200 some odd apps (most of those just wasting 1's and 0's).... I got the 64G because I could. If it was only the 32 and 128 and a $100 spread between them... I dunno. I probably would have got the 128 version, only 'because moar', but I don't think I would have had any issue with the 32.