"without making other areas worse"
Puh! Lucky us! These days you have to be very grateful to hear that Apple did not **** up another thing during an update!
----------
This is plain wrong: even if you stored any given iOS install image anywhere you still have to go through iTunes installation process - and that is the time where iTunes goes online and checks the signature of that install image.
And guess what: Apple won't accept install images older than iOS 8. So you cannot even "restore" an iDevice running on iOS 7 to a clean iOS 7.
----------
There is a well-known "trick" though to install previous versions of apps (which were still running on that older iOS you want it for), even if you did not purchase (or get for free) them previously:
* you simply get the latest version through iTunes.
* At this point you get the latest of greatest version of the app (possibly only running on iOS 8)
* The net result is: that app now "belongs to you"
* Now you start the App Store on your actual iDevice running a lower (non-supported) iOS version and try to install that very app
* A message will tell you that the latest app is not available for you iOS version, but you can install an older version of that app (if available!)
Puh! Lucky us! These days you have to be very grateful to hear that Apple did not **** up another thing during an update!
----------
If you saved your blobs on older iOS/Devices you still can.
This is plain wrong: even if you stored any given iOS install image anywhere you still have to go through iTunes installation process - and that is the time where iTunes goes online and checks the signature of that install image.
And guess what: Apple won't accept install images older than iOS 8. So you cannot even "restore" an iDevice running on iOS 7 to a clean iOS 7.
----------
If you already have the apps you need you may be fine, but if you sell or gift the device to someone else they will have a crippled device with an artificially restricted selection of apps.
There is a well-known "trick" though to install previous versions of apps (which were still running on that older iOS you want it for), even if you did not purchase (or get for free) them previously:
* you simply get the latest version through iTunes.
* At this point you get the latest of greatest version of the app (possibly only running on iOS 8)
* The net result is: that app now "belongs to you"
* Now you start the App Store on your actual iDevice running a lower (non-supported) iOS version and try to install that very app
* A message will tell you that the latest app is not available for you iOS version, but you can install an older version of that app (if available!)