This is the same Apple that introduced 4k recording with the iPhone 6S and still sell 16GB models.and it's obviously storage related, for anyone willing to a) google and b) do some math c) not limit themselves to the one thing they felt like thinking about (storage speed? yet you skip right past capacity so you can what, make a point about being upset about a feature you clearly know nothing about, and thus wont even be using?)
luckily those guys did it for us. but in the interest of finding multiple sources, another site's math says a 128gb device would be full within 5 hrs of 4k30 recording, no mention of HDR
that first link also did math to get a bit rate of 31.1mbps. apple's lowest tier prores format boasts a bit rate of 45mbps, and goes up to 105mbps from there.
this move makes sense from a "we're trying to make sure your device isnt going to slow to a crawl once you wipe out the usable space for file swapping."
having done a great deal of 4k60DV recording, and also video editing, this is hardly a bad choice to implement.
"waaaah just let the user have it anyway!"
user: "waaaaaaaah apple why is my phone always out of space"
besides, anyone recording a great deal of 4k60HDR videos on their iphone already took into account how much storage space this stuff takes, making this a non issue.
No excuses for Apple. I bet the engineering team did design the 13Pro with the faster storage in mind. But then the accountant team probably wants to maximize Apple's margin si the need to keep 128GB as base model by sacrificing the feature.