Next time, you have to pay for a backup solution!
Apple clearly can take your data from phone & use the PC/Mac as a temporarily "buffer" storage until update is completely successful , even in case of problems at least data is safely reside on PC/Mac (encrypted of course) so you don't lose.
But alas, Apple way is to pay for it!
Next time: prepare to backup through various free backup service like your own cloud & time machine (local server) (check nextcloud) , you can also use google photo for backup (free 15GB) , for other data you can spread some through dropbox, onedrive ..etc.
Be warned though, Apple's ecosystem from ground up is constructed around forcing customers pay for service, so restrictions will always -always- apply.