You can argue that's it not technically inconcievable but if anyone were to implement it this way, it would likely become a nightmare of usability and broken ex

You can argue that's it not technically inconcievable but if anyone were to implement it this way, it would likely become a nightmare of usability and broken expectations. If I'm 'downloading' or otherwise 'exporting' data from any service, I expect it to be all the data. Not a diff wrt the last time I clicked the button.