I restore old leather saddles. That is, I form new leather tops and rivet them on. So my solution to ruined leather is: new leather! I realize that's not what we're talking about here.
But I've had a lot of leather saddles where the leather has reached the end of its useful life, and in my experience there's nothing you can do to rejuvenate it. As it ages, it loses its ability to stretch, and once this is lost, it is prone to cracking and tearing. I've had a saddle rip right in half while I was riding. So while Proofide, wax, neatsfoot oil, etc all have their place in saddle maintenance, their purpose is to preserve the leather, to protect it from water, and in general to slow its deterioration. Too much will make a saddle too soft, allowing the leather to stretch permanently, and then there's really nothing you can do to unstretch it.
On the other hand if you don't treat your saddle and leave it out in the rain repeatedly, the leather will shrink and harden, and then there's nothing you can do to soften it up again. You can soak it in water and soften it temporarily, and reshape it while its wet; but when it dries, it will be hard again. You can rub Proofide and neatsfoot oil into it and soften it up a bit, but you cannot undo whatever damage has been done. If the leather has become brittle, it will remain brittle. In short, whatever you do to your saddle, you can change the rate at which it ages, but you can't make it young again.
