Maybe we are talking about 2 separate things, I believe. Filetype of save file does not have to be different for different version, right? Size concerns, is something I understand, however that's not the point of this thread. Save files are. And why do you have to download the latest version of past gen title, upload it to the cloud (presumably which is also payed), so you can load it up? Also it does not really work at all times, as far as I believe, some devs cannot be fucked with this file transfer and only gives you MP progress or some incomplete save file, which does not contain everything (on top of my head Metro Exodus and Doom Eternal).
Like I said try to propose this solution to some project manager or client and you would laughed at/fired.