Depends on your definition of 'heavy user.' I've had a 32GB Nexus 6 for 2 years now (which also records at 4k) and have managed just fine with plenty of music stored on device, usually 1 or 2 games installed (no need for a whole library) as well as a couple of movies. Sure, if you have 128GB, you can store a lot more, but then.... wouldn't you really need 256GB? 512GB? 1TB? There is a minimum point which can be functional for the large majority, and I think that 32GB hits that point after affording space for the OS,etc. While some users - primarily those looking to record large sections of 4k video in short spans - might actually need 128GB, beyond that it's a matter of whether it's worth it to the individual to pay the premium in order to make managing avaialble space a lesser concern.
That said, I'd have gotten the 128GB if it had been available, but with it being an extra month out, have gone with the 32GB and am not concerned for it.
As to Swappa, you'll nearly always see more of the lower storage units on sale than higher for the simple reason that more of them are sold in the first place. But as to any on there now, it's likely people trying to make a quick buck flipping theirs due to the extreme demand and unavailability in the first place.