I posted recently on this topic - it's a good question. It's totally based on personal use cases.
Last year I bought a Note 5 with 32GB and although all of my phones for years have been 32GB, I thought it would be insufficient. So I bought an OTG 64GB drive for all of videos and music I was dreaming about storing on my phone.
Same thing with my car. I have a 64GB micro flash in my dashboard for "all the music" I want to take with me.
My Note 7 has 64GB.
Fact is, I never once used that OTG drive on either of my Notes, my car's flash drive remains 75% empty, as is my Note 7. Turns out I really don't want to have to navigate through thousands/thousands of songs on my phone or in my car - what a PIA. And I tend to curate, I add stuff I want to listen to now, and I delete stuff I no longer want to listen to. On my phone, all photos are automatically pushed to Google Drive. Perfect.
With all that said, I bought the 32GB Pixel XL, AND a USB-C OTG adapter for my 128GB microSD.
OCD I guess.
Nice to know that I can bring 30 movies and 10,000 songs on my next flight.
Anyone want to bet that I don't once use that 128GB storage with the Pixel?