I have the encrypted file of the backup through itunes on my pc, and i’ve been using it as pseudo storage since it had over 100gb of pictures and videos and my phone (128gb) needed more space for other files. But until I can get a phone with bigger storage or figure out something else, the file is in cold storage for now.
Would the file be obsolete in a few months if I keep it as is?
You can, so long as the receiving device is running an iOS version equal or newer to the backup, it can be restored. Technically I could even make a backup from my 1st generation iPhone and restore it to an iPhone 15.