Every app can write anything to external storage this has existed for a while for the iPad and 3rd party apps.
Apple is making a cheaper, lower capacity option with the ability to add your own unrestricted storage and you complain about… nothing really.
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If you’re a professional shooting 4K 60 ProRes video you likely have the budget to upgrade the storage (or shoot with a real professional grade camera.) it’s highly unlikely you could have a good user experience with 128GB (minus the OS and all your other personal garbage) filling up in minutes shooting in this mode. The lower capacity models aren’t designed for those specific use cases in mind, and it’s cool they’ve found a way around that limitation through the use of external attached storage.
People just looking for sky to yell at ;)