Storage is one of those things that what is good for the goose may not actually be good for the gander.
it’s really simple if you like to download music or videos to watch later in non-reception areas, then you want 256 GB.
Personally 64 GB is enough for me because I keep all my music and anything else I can in the cloud I don’t download it to my phone.
I take pictures and videos off my phone regularly, name them, and store them on my computer, a back up drive, and in Google cloud. If I let things get any greater than 50 GB I’ve got a lot of work on my hands to name everything and recall all the pictures that I like to organize.
I purchased 50 GB of iCloud space just in case I should ever lose my data on my phone before transferring over to my computer and back up for safety.
So technically I never let it get past 50 GB, no matter if their 4K videos or pictures and any storage greater would be a waste for me on my phone. I stream Apple Music, Netflix etc from the cloud.
Having said that, there’s no doubt in this day of age that an Apple phone you pay $1000 for shouldn’t have 128 GB of storage default.
Perhaps you can get away with 64 GB of space on the lower end models that you pay $700 would be the wiser plan for consumers and Apple competition.
Otherwise it just boils down to plain greed like others have pointed out. Not to mention the ridicule of android users who get 128 GB default on even their cheap Crap phones.